CS ASSIGNMENT #3

To grow at all, gems like beryl, tourmaline, and kunzite need tiny amounts of elements that are so scarce that it's difficult to express their concentration as a percentage (parts per hundred). Instead they're described in terms of parts per million. Chromium, the element that makes emerald green and ruby red, is only present in

100 parts per million in the earth's crust.

It's easiest to visualize a contact twin as a single crystal that's been cut in half, with one half rotated

180°. Each half is identical, but the crystal's structure appears to be reflected in the plane where the two halves come together. This twinning plane marks the location where the direction of the gemstone's crystal structure changed during growth.

Amorphous

Amorphous No crystal structure. Most of these are either cooled too quickly to crystallise - such as obsidian or moldavite, or are organic - such as amber. Common Amorphous Minerals: Amber Moldavite Obsidian Opal

Smaller crystals are formed

Crystal size relates directly to growth conditions. A solution that is highly saturated in nutrients produces many small crystals resulting in a mass called an aggregate. rapid cooling produces similar results a mass of tiny crystals.

Larger crystals are formed

Crystal size relates directly to growth conditions. I f cooling is slow, individual crystals have more time to develop. In addition, a lower concentration of nutrients leads to fewer crystallization center which also results in larger single crystals. Pegmatite are the best examples of this type of growth

_________ occurs when the twinning planes aren't parallel, and it results in a wheel-shaped crystal.

Cyclic twinning. Like contact twins, penetration and cyclic twins occur during crystal growth, so they're also examples of primary twinning.

What features can help you recognize different rough gemstones?

Each rough gem has a characteristic shape or range of possible crystal shapes depending on the conditions it grows under. This characteristic shape is called its habit. It results from an interaction between the growing crystal and its environment. Shape can help you tell rough ruby from rough spinel, and either one from rough red garnet or tourmaline. These clues are extremely valuable to people who buy and sell rough gems for a living. Knowledge of rough helps them avoid costly mistakes because they know that not every red gem is a ruby. (As you'll learn in Assignment 5, dishonest traders sometimes create copies of rough gem shapes and try to sell them as authentic gem rough.) Knowledge of rough also alerts a buyer to a rough gem material's potential. Knowing that many rough rubies occur as flattened crystals helps a buyer understand why so many cut rubies have shallow proportions.

You can't always explain differences in gemstone density by judging how heavy their component elements are. Diamond, for example, is made only of carbon, which is one of the lighter elements. But diamond's specific gravity is greater than that of quartz, which is made of two heavier elements: silicon and oxygen. There are two reasons for this:

First, the high pressures of diamond formation press the carbon atoms tightly together. Second, very strong chemical bonds between the carbon atoms hold them there. The net result is simply that more atoms are packed into the same amount of space. So density isn't just affected by the type of atoms. How they're packed matters, too.

How do physical properties affect a gem's durability?

Hardness, toughness, and stability combine to affect durability.

#3 Hexagonal System

Hexagonal System Not all crystals have cube-shaped unit cells. Some—like beryl, corundum, and quartz—have six-sided crystals and hexagonal unit cells. Beryl is more symmetrical than the others, so it's classified in the hexagonal crystal system. Three out of the four axes are in one plane, of the same length, and intersect each other at angles of 60 degrees. The fourth axis is of a different length and intersects the others at right angles.Based on a hexagonal (6-sided) inner structure. Crystal shapes include: Four-sided prisms and pyramids Twelve-sided pyramids Double pyramids Common Hexagonal Crystals: Apatite Aquamarine Beryl Cancrinite Emerald Goshenite Morganite Sugilite Zincite

_________ is an aggregate that's also a rock because it's composed of more than one mineral. It usually contains lazurite, pyrite, and calcite, but it might also contain other minerals.

Lapis lazuli

#6 Monoclinic System

Monoclinic System The angles between the flat faces on the flattened brick are still at 90 degrees. Now, distort the brick so that one set of angles becomes less than 90 degrees. You've made a much less symmetrical unit cell. Crystals shaped like this belong to the monoclinic crystal system. Kunzite is a gem in the monoclinic system. There are three axes, each of different lengths. Two are at right angles to each other and the third is inclined.Based on a parallelogram inner structure. Crystal shapes include: Basal pinacoids and prisms with inclined end faces Common Monoclinic Crystals: Azurite Chrysocolla Diopside Epidote Gypsum Hiddenite Howlite Kunzite Lazulite Moonstone Muscovite (Mica) Petalite Serpentine Spodumene Staurolite Vivianite

#5 Orthorhombic System

Orthorhombic System Now, take the brick and flatten it so that all three directions have different measurements. Crystals built from unit cells shaped like this are even less symmetrical, and they belong to the orthorhombic crystal system. Topaz and peridot both belong to this system. (Rhombic System)Three axes, all of different lengths, are at right angles to each other.Based on a rhombic (diamond-shaped) inner structure. Crystal shapes include: Pinacoids Rhombic prisms Pyramids Double pyramids Common Orthorhombic Crystals: Alexandrite Andalusite (Chiastolite) Celestite Chrysoberyl Chrysoberyl Danburite Dumortierite Enstatite Hemimorphite Iolite Tanzanite Topaz Zoisite

What are the different ways a gem can break?

Perhaps the most important component of toughness is the way a gem reacts to stress when it breaks. The different ways a gem breaks are called cleavage, parting, and fracture.

Recognizing roughs hidden structures

Tabular: Squat and flat, like many corundum crystals. Prism or prismatic: Columnar, with 3, 4, 6, 8, or 12 parallel faces. Many aquamarine and tourmaline crystals are prismatic. Euhedral (Yoo-HEE-dral): Well formed, with sharp crystal faces, like most gems from pegmatite pockets. Anhedral (Ann-HEE-dral): Lacking obvious crystal faces, like many gems that have been tumbled in rivers. Striations: Horizontal (quartz, corundum) or vertical (tourmaline, topaz) growth markings on a crystal. Pyramid: Shape with equal triangular faces that meet in a point. Bipyramid: Shape with two pyramids back-to-back. Seen in sapphires, for example. Squat, flattened crystals with triangular surface features on the top surface are characteristic of ruby. Sapphire crystals typically have two sets of six triangular faces joined back-to-back. Their linear markings run horizontally across the crystal faces. Star-shaped chrysoberyl twins are common in the gem gravels of Brazil. Imperial topaz crystals have a diamond-shaped cross-section, vertically grooved sides, and a chisel-shaped top. Spinel often occurs as well-formed eight-sided (octahedral) crystals. They're common in the gem gravel of Mogok, Myanmar. Garnet occurs as 12-sided (dodecahedral) crystals. Most garnet crystals in gem gravel have been worn to a rounded ball-like shape.

#2 Tetragonal System

Tetragonal system Scapolite is a tetragonal gem that usually crystallizes as a four-sided columnar crystal with a pyramid termination. If you stretch a cube along one dimension, it results in a brick shape, longer in one direction than the others. Crystals with unit cells shaped like this belong to the tetragonal crystal system. This internal structure can affect the physical properties of a gem—such as hardness—or the way a gem interacts with light. The colors of the gem might vary with viewing direction as well. Zircon is an example of a tetragonal gem. Two axes are of equal length and are in the same plane, the main axis is either longer or shorter, and all three intersect at right angles.Based on a rectangular inner structure. Crystal shapes include: Four-sided prisms and pyramids Trapezohedrons Eight-sided and double pyramids Icosi-tetrahedron (pyrite, sphalerite) Hexacisochedron (pyrite). Common Tetragonal Crystals: Anatase Apophyllite Chalcopyrite Rutile Scapolite Scheelite Wulfenite Zircon

How does crystal shape affect the way a stone is cut?

The gem cutter (lapidary) examines a rough tanzanite while rotating it. Will the rough yield one big stone or many smaller ones? Which is the best face-up direction for the best color? Which choice will bring the most profit? For instance, topaz crystals are often long and thin. To retain weight the lapidary might cut long, thin ovals or elongated rectangular step cuts. For every well-proportioned imperial topaz you'll see, there will be 10 or more long, thin ovals or pears. Inner structure can also influence cutting decisions. Because of topaz's cleavage plane is parallel to its base, it has a tendency to break parallel to the base of the crystal, stones are very vulnerable to knocks, or even the pressure of setting. This can make these attractive stones difficult to use and difficult to set. To make up for this weakness and minimize breakage during fashioning, the cutter tries to orient a topaz's table facet at a slight angle.

How does a rough crystal get its start?

The shape of a rough crystal dictates the shape and size of the finished gem. Because ruby rough is so expensive, a cutter would probably fashion a single shallow stone from this flattened crystal (left). It would result in a gem very similar to the 5x4-mm Thai ruby (right). To produce the largest stone possible, the cutter was forced to cut a very shallow gem. It's so shallow that the text is visible through it. The well-formed faces on a glittering quartz crystal are an outward expression of its inner order. They reflect the internal repetition of tiny identical geometric building blocks. Each building block is called a unit cell. The unit cell is the mineral's "signature"—its basic identity. It consists of the smallest group of atoms with the same chemical composition and crystal structure as the gem material as a whole. All crystals are composed of unit cells in repeating three-dimensional arrangements. The same factors control the growth of all crystals, from the largest—like the spectacular crystals formed in pegmatite—to the smallest—like the crystals in chalcedony, which can be seen only under the highest magnification. No matter what type of crystal is forming, and whether it's synthetic or natural, the crystallization process is basically the same: Atoms come closer and closer together until they finally form connecting bonds. If conditions are right—the right elements and correct temperature and pressure—and there's sufficient space and time, the arrangement of the atoms and their connecting bonds reflects the symmetrical structures that are typical of crystal growth.

#1 Cubic System

The unit cell of a cubic crystal is shaped like a cube. You can stack cubes to build a bigger cube, or you can stagger them to build an octahedron, or even a 12-sided figure called a dodecahedron. Rough garnet is typically this shape. Cubic crystals are more symmetrical than the crystals in other systems. Their external symmetry is made possible by their highly ordered structure at the atomic level. This atomic symmetry gives them fewer directional differences in their physical and optical properties. Also known as the isometric system. All three axes are of equal length and intersect at right angles. Based on a square inner structure. Crystal shapes include: Cube (diamond, fluorite, pyrite) Octahedron (diamond, fluorite, magnetite) Rhombic dodecahedron (garnet, lapis lazuli rarely crystallises) Icosi-tetrahedron (pyrite, sphalerite) Hexacisochedron (pyrite). Common Cubic Crystals: Diamond Fluorite Garnet Gold Pyrite Silver Spinel

Why do different colored stones with similar dimensions have such different carat weights?

These gems appear similar in size, but the aquamarine (left) weighs 20.01 cts. and the topaz (right) weighs 26.37 cts. This is because aquamarine's specific gravity is less (2.70) than topaz's (3.60). SG differences are caused by differences in the type and packing of the atoms that build each gem.

Toughness is described as

Toughness is described as exceptional (jadeite and nephrite), excellent (corundum), good (quartz and spinel), fair (tourmaline), or poor (feldspar and topaz).

#7 Triclinic System

Triclinic System If you distort the brick so no angle is 90 degrees, you've arrived at the least symmetrical crystal unit cell. Rhodonite and labradorite feldspar have unit cells of this type and belong to the triclinic crystal system. All three axes are of different lengths and inclined towards each other.Based on a 'triclinic' inner structure, meaning 'three inclined angles'. Crystal forms are usually paired faces. Common Triclinic Crystals: Amazonite Aventurine Feldspar Kyanite Labradorite Rhodonite Turquoise

#4 Trigonal System

Trigonal system (Rhombohedral System) Corundum, tourmaline, and quartz have less symmetrical unit cells and belong in the trigonal (sometimes called rhombohedral) crystal system. - Axes and angles in this system are similar to the Hexagonal System, and the two systems are often combined as Hexagonal. In the cross-section of a Hexagonal crystal, there will be six sides. In the cross-section of a Trigonal crystal there will be three sides.Based on a triangular inner structure. Crystal shapes include: Three-sided prisms or pyramids Rhombohedra Scalenohedra Common Trigonal Crystals: Agate Amethyst Aventurine Calcite Carnelian Citrine Hematite Jasper Phenakite Quartz Rhodochrosite Rose Quartz (rarely crystallises) Ruby Sapphire Smoky Quartz Tigers Eye Tourmaline

_________ present a problem to the cutter. Traditional diamond cutting methods make use of a cutting blade coated with diamond crystals. A cutter can saw all the way through an untwinned rough diamond by following the diamond's best sawing direction. If the direction of the diamond's crystal structure changes, as it does in a twinned diamond, the best sawing direction isn't continuous through the stone. Colored stones are softer than diamond, so twinned colored stones don't present the same cutting difficulties as twinned diamond crystals.

Twinned diamond crystals. Traditional diamond cutting methods make use of a cutting blade coated with diamond crystals. A cutter can saw all the way through an untwinned rough diamond by following the diamond's best sawing direction. If the direction of the diamond's crystal structure changes, as it does in a twinned diamond, the best sawing direction isn't continuous through the stone. Colored stones are softer than diamond, so twinned colored stones don't present the same cutting difficulties as twinned diamond crystals.

What's the difference between hardness and toughness?

While hardness is resistance to scratching, toughness is resistance to breaking and chipping.

Gem minerals that are made up of dense, closely packed masses of tiny, randomly oriented crystals are called _________.

aggregates.

Peridot is a gem that's susceptible to

etching by certain chemicals. Chemicals used during jewelry repair damaged the surfaces of this peridot.

If a crystal forms at high temperatures, its structure might change as it cools. Twinning can occur as the mineral rearranges its structure to conform to the lower temperature conditions. This often happens in

feldspars.

If a gem doesn't have planes of weakness like cleavage or parting, the pattern of breakage is different. The term _________ is used to describe a break in a gem other than cleavage or parting. The most common type of fracture in transparent gems yields a characteristic surface pattern of concave ridges, like a clamshell. This type of fracture is called conchoidal fracture, and it occurs in glass as well as in quartz, opal, peridot, and many other colored gems. Jade's structure of tiny interlocking crystals gives its fractures a granular surface appearance, because the breaks occur within or between individual crystals. Rocks like lapis lazuli also have a granular fracture surface because they're made up of a mixture of different minerals, each of which reacts differently to stress.

fracture. The most common type of fracture in transparent gems yields a characteristic surface pattern of concave ridges, like a clamshell. This type of fracture is called conchoidal fracture, and it occurs in glass as well as in quartz, opal, peridot, and many other colored gems. Jade's structure of tiny interlocking crystals gives its fractures a granular surface appearance, because the breaks occur within or between individual crystals. Rocks like lapis lazuli also have a granular fracture surface because they're made up of a mixture of different minerals, each of which reacts differently to stress.

If trace elements are not already incorporated during growth, the atoms of a trace element might enter the crystal lattice later. This requires

high temperatures because the energy provided by heat causes the trace elements to migrate into the crystal lattice. The high temperatures can exist during formation, or they can be introduced by heat treatments used to improve color or clarity. As the crystal cools, trace elements might stay trapped in the crystal lattice. If there's not enough room for them in the cooling crystal's structure, they're forced out. This can lead to the formation of inclusions.

The individual crystals in aggregates come in a variety of sizes. They might be readily visible, like the radiating crystals that make up rock crystal. They can be _________, like nephrite, jadeite, and quartzite, where the crystals are visible under magnification. They can also be _________, like chalcedony and turquoise, where only the highest power magnification (beyond the power of a standard gemological microscope) can detect individual crystals.

microcrystalline, cryptocrystalline

A gem's density and specific gravity depend

on the types of atoms it's made of, and how closely they're packed together.

Crystals are the raw material for all these gems. Most rock and mineral crystals are formed from the eight most abundant chemical elements in the earth's crust:

oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium.

Sometimes a rough ruby or sapphire crystal has three sets of parallel lines aligned with certain crystal faces. These lines of weakness are associated with twinning, a common feature of both. The crystal might break along these lines if it's subjected to enough pressure. This is described as

parting. Parting is different from cleavage. Parting is a flat break caused by heavy concentrations of included minerals, which may be associated with twinning planes. Cleavage, on the other hand, is caused by a gem's own atomic structure. If you examine enough faceted rubies and sapphires with a microscope, you'll notice that thin needles (composed of an aluminum mineral called boehmite) are associated with twinning planes. These needles grow during twinning when the crystal is subjected to pressure. If enough boehmite needles are concentrated along the twinning planes, this creates weak areas where there is less corundum present than elsewhere. If the crystal is stressed along these inclusion-rich layers, this is where it will break. Parting is most common in very highly twinned or included crystals that are used for cabochons or beads. These stones rarely achieve a good polish because tiny fragments of the crystal are pulled out during polishing.

Some twinned crystals look as though two crystals have grown through one another. Fluorite and quartz show this type of twinning, which is known as

penetration twinning.

Some crystals might start out untwinned but become twinned when the rocks they formed in are altered by pressure during metamorphism. This type of mechanical stress causes the closely spaced parallel twin planes often seen in corundum and feldspar crystals. This repeated twinning is known as _________ or lamellar twinning.

polysynthetic or lamellar twinning.

Gem materials that are commonly twinned include

quartz, spinel, corundum, feldspar, and chrysoberyl. Twinned crystals often look entirely different from untwinned gems of the same species.

As the crystal cools, excess titanium and oxygen cannot be accommodated in the sapphire's crystal lattice, so they combine to form

rutile. The rutile forms fine needles, called "silk." The needles are aligned parallel to the sapphire's crystal directions. If enough needles are present, they might give rise to a six-rayed star.

SG is listed as a number. Amethyst's SG is 2.65; zircon's can range up to 4.70. The number tells you how much a mineral weighs compared with the _________. This means that amethyst is just over 2.5 times as heavy as the same volume of water. Zircon is nearly 5 times as heavy as an equal volume of water.

same volume of water.

Twinning doesn't happen only while crystals are growing. Changes in environment after a crystal has formed might also cause twinning. In this case, it's called _________, and it can be caused by cooling temperatures or the pressure of metamorphism.

secondary twinning

Density (heaviness of an object compared to its size) can be measured by comparing the weight of the gem with an equal volume of water. This results in a ratio known as

specific gravity (abbreviated SG). SG can be a helpful identifier for all gemstones.

A gem's ability to tolerate the effects of light, changes in temperature, or the action of chemicals is called

stability. Gems with poor stability, including many treated gems, require extra care in wear and cleaning.

Peridot, tourmaline, tanzanite, and some garnets react poorly to sudden changes of temperature. Fractures can result from the careless application of a jeweler's torch during repair procedures. Damage of this type is described as

thermal shock. It can occur when the temperature change is from hot to cold or from cold to hot.

As a crystal grows, some chemical elements that aren't necessary for its growth can become trapped in its crystal structure. Since they're present at the atomic level, they're not considered inclusions. They're called

trace elements, and they can affect a gem's appearance in many ways.

Sometimes a crystal's growth is interrupted early in its formation by a change in its growing environment. Then, when growth begins again, it might not follow the same direction as before, even though it still has the same form as the existing crystal. This change in a gem's crystal direction is known as

twinning.

Each building block is called a

unit cell. The unit cell is the mineral's "signature"—its basic identity. It consists of the smallest group of atoms with the same chemical composition and crystal structure as the gem material as a whole.

How do inclusions and trace elements become part of a gem?

An inclusion is a characteristic contained within a gemstone, or one that reaches its surface from the interior. Inclusions are a byproduct of natural crystal growth. No gem grows in isolation, and almost every cut gem carries some trace of the different minerals it grew with. An inclusion can be a distinct mineral crystal of any size, shape, or color. It can be a cloud of minute particles or a hollow cavity filled with a combination of a liquid, a gas, and sometimes also a solid crystal. Color zoning (caused by variations in coloring impurities) and fractures (caused by stress to the crystal) are also classified as inclusions. Some inclusions form at the same time as the gem. The conditions that a crystal grows in might allow the growth of many other mineral species at the same time. If the gem crystal develops faster than the minerals around it, it might enclose them completely, preserving them as inclusions. The "brassy" metallic pyrite crystals in some emeralds are good examples. The crystals grew along with the gem, so their crystal faces are sharp and angular. Gems that grow in mineral-rich watery solutions often enclose liquids. Topaz, emerald, and amethyst can have abundant liquid inclusions—hollow, fluid-filled cavities. Sometimes they also contain a gas or a solid, or both. When only two of those things are present—typically liquid and gas—they're described as two-phase. If all three are present, it's described as three-phase.

The seven crystal systems are a method of classifying crystals according to their

atomic lattice or structure. Gems like diamond, spinel, garnet, and fluorite crystallize in the cubic crystal system (sometimes called the isometric system). Cubic minerals have the highest symmetry. A well-formed spinel or diamond crystal has a highly ordered and symmetrical arrangement of crystal faces. This is because their unit cells are also highly symmetrical. The atomic lattice is a three dimensional network of atoms that are arranged in a symmetrical pattern. The shape of the lattice determines not only which crystal system the stone belongs to, but all of its physical properties and appearance. In some crystal healing practices the axial symmetry of a crystal is believed to directly influence its metaphysical properties. For example crystals in the Cubic System are believed to be grounding, because the cube is a symbol of the element Earth. There are seven crystal systems or groups, each of which has a distinct atomic lattice.

If the crystal's structure has weaker bonds or fewer bonds in certain directions, then stress will cause breaks in those directions first. If you strike a faceted tanzanite or kunzite sharply, it might break along planes determined by the gem's atomic structure. This type of break is called

cleavage, and the weaker directions are called cleavage planes. Cleavage is a smooth, flat break in a gemstone parallel to its cleavage planes. Because a gem has many parallel cleavage planes, broken surfaces might have a step-like appearance. "Cleavage" also refers to the property of a gem that allows the break to occur. Cleavage is part of a mineral's structure. Each and every example of that mineral has the same hidden weakness. For instance, in diamond, the chemical bonds between individual carbon atoms are all the same type and strength, but in cleavage directions there are simply fewer bonds.

If twinning happens during a crystal's original growth period, it's known as primary twinning. In one type of primary twinning, the twinned crystals look like one half is a mirror image of the other. These are known as

contact twins, and they are seen so often in spinel that they are sometimes also called spinel twins.

Cracks in gem-quality opal—called _________—are probably a result of water loss from the gem's structure over the course of time. Relatively few opals craze like this.

crazing

Gems like diamond, spinel, garnet, and fluorite crystallize in the

cubic crystal system (sometimes called the isometric system). Cubic minerals have the highest symmetry. A well-formed spinel or diamond crystal has a highly ordered and symmetrical arrangement of crystal faces. This is because their unit cells are also highly symmetrical. The unit cell of a cubic crystal is shaped like a cube. You can stack cubes to build a bigger cube, or you can stagger them to build an octahedron, or even a 12-sided figure called a dodecahedron. Rough garnet is typically this shape. Cubic crystals are more symmetrical than the crystals in other systems. Their external symmetry is made possible by their highly ordered structure at the atomic level. This atomic symmetry gives them fewer directional differences in their physical and optical properties.