“Crystal 1,”?photo by Brenda Clarke. Licensed under CC By 2.0.
Crystal Systems
All minerals form in one of seven crystal systems: isometric, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal. Each is distinguished by the geometric parameters of its unit cell, the arrangement of atoms repeated throughout the solid to form the crystal object we can see and feel.
For example, an isometric or cubic crystal has a cube as its unit cell. All its sides are equal in length and all its angles are right angles. Well-known gems in this system include diamonds, garnets, and spinels.
The isometric crystal system has three axes of the same length that intersect at 90o angles.
On the other hand, a triclinic crystal has all sides of different lengths and none of its angles are right angles. These geometric variations mean triclinic crystals can take on many intricate shapes. Well-known gems in the triclinic system include labradorite and turquoise.
None of the axes in the triclinic system intersect at 90o and all are different lengths.
Non-Crystalline Solids
Some objects may appear to be crystals to the naked eye, but outward appearances can be deceiving. For gemologists, the atomic structure of the object is the determining factor. Not all objects with regular geometric faces are crystals, nor are all solid materials crystals.
Amorphous Solids
Glass, for example, has a non-crystalline, amorphous atomic structure. Although glassmakers can pour and harden glass into geometric shapes, its atomic structure remains unchanged.
People commonly refer to some glassware, such as this, as crystal. This is an accepted popular use of the term. However, scientifically speaking, these objects aren’t crystals. Photo?by liz west. Licensed under CC By 2.0.
Polycrystalline Solids
Water that hardens into a single large snowflake is, in fact, a crystal. It crystallizes as it cools, freezes, and moves through the atmosphere.
“Snowflake-23,”?photo by Yellowcloud. Licensed under CC By 2.0.
However, water that hardens into a cube in your freezer’s ice tray isn’t a crystal. Ice cubes, rocks, and common metals are examples of polycrystalline materials. They may contain many crystalline objects. (In the case of ice cubes, they may contain actual ice crystals). Nevertheless, you can’t describe the entire ice cube as having a uniform crystalline structure.
“Frozen Ice Cubes IMG_1021,”?photo by Steven Depolo. Licensed under CC By-NC 2.0.
Cryptocrystalline or microcrystalline rocks consist of microscopic crystals, but, again, those rocks lack a uniform crystalline structure. Some cryptocrystalline materials, such as chalcedony, find use as gem materials in jewelry or decorative objects.
The Origins of Crystals
Most crystals have natural origins. They can form through inorganic means, such as geological processes within the earth. Others form through organic processes within living creatures.?For example, some human kidney stones consist in part of weddellite crystals.
Weddellite occurs at the bottom of the Weddell Sea near Antarctica. It can also be found passing very painfully through urinary tracts. Here, you can see tetragonal weddellite crystals on an amorphous stone. “Surface of a Kidney Stone,”?photo by Kempf EK. Licensed under CC By-SA 3.0.
Laboratories can also create crystals artificially. For example, cubic zirconia, a synthetic?gem material, forms with a cubic crystal structure when zirconium and zirconium dioxide are superheated. The resulting material commonly finds use as a diamond imitation or simulant.
Colorless cubic zirconia gems often serve as diamond imitations. However, labs can also synthesize this material in many colors, including multicolor specimens like this one. Photo?by Michelle Jo. Licensed under CC By 3.0.
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