Colored Stones: Crystal Systems and Gem Identification

Crystal Systems and Identification

• Crystalline minerals are classified into 7 crystal systems based on the symmetry of their unit cells.
• Recognizable crystal shapes of rough gemstones can help identify them.
• The exterior shape of a crystal often indicates its symmetry at an atomic level.

Factors Affecting Gemstone Shapes and Sizes

• Shapes and sizes of rough gem crystals affect the shapes and sizes of finished stones.
• Rarity of gems is due to the requirement of rare chemical elements or unique geological conditions, or both, to form.

Physical Properties of Gems

• 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 occur due to a change during crystal growth, or from cooling or pressure after the crystal has grown.

Fracture and Inclusions

• Different ways a gem breaks are called cleavage, parting, and fracture.
• Inclusions in a colored stone can help identify it as natural or man-made, untreated or treated.
• A gem's crystal structure determines its reaction to external stress.
• Large inclusions can dramatically affect beauty, value, and durability.