CS Book 1_AS- 3 Physical Properties.pdf

Colored Stones 3 KEY CONCEPTS Crystalline minerals are classified into seven crystal systems, depending on the symmetry of their unit cells. Many rough gemstones have recognizable crystal shapes that can help identify them. The exterior shape of a crystal often indicates its symmetry at an atomic le...

Colored Stones 3 KEY CONCEPTS Crystalline minerals are classified into seven crystal systems, depending on the symmetry of their unit cells. Many rough gemstones have recognizable crystal shapes that can help identify them. The exterior shape of a crystal often indicates its symmetry at an atomic level. The shapes and sizes of rough gem crystals affect the shapes and sizes of finished stones. Some gems are rare because they require rare chemical elements or unique geological conditions, or both, to form. A gem’s density and specific gravity depend on the types of atoms it’s made of, and how closely they’re packed together. Twinning can result from a change during crystal growth, or from cooling or pressure after the crystal has grown. The different ways a gem breaks are called cleavage, parting, and fracture. Inclusions in a colored stone can help identify it as natural or manmade, untreated or treated. A gem’s crystal structure determines its reaction to external stress. Large inclusions can dramatically affect beauty, value, and durability. 134