Gems and Their Physical Properties PDF

Gems and Their Physical Properties Key Terms Aggregate—A mass of tiny, randomly oriented crystals. Parting—A flat break in a gemstone caused by concentrated included minerals parallel to a twinning plane. Cleavage—A smooth, flat break in a gemstone parallel to planes of atomic weakness, caused by we...

Gems and Their Physical Properties Key Terms Aggregate—A mass of tiny, randomly oriented crystals. Parting—A flat break in a gemstone caused by concentrated included minerals parallel to a twinning plane. Cleavage—A smooth, flat break in a gemstone parallel to planes of atomic weakness, caused by weak or fewer bonds between atoms, or both. Specific gravity (SG)—Ratio of the weight of a material to the weight of an equal volume of water. Conchoidal fracture—A curved and ridged fracture in a gemstone, extending from the surface inward. Stability—How well a gemstone resists light, heat, and chemicals. Cryptocrystalline—An aggregate made up of individual crystals detectable only under very high magnification. Thermal shock—Damage caused by sudden, extreme temperature changes. Density—How heavy an object is in relation to its size. Three-phase inclusion—A hollow cavity in a gem filled with a liquid, a gas, and one or more crystals. Durability—A gemstone’s ability to withstand wear, heat, and chemicals. Fracture—Any break in a gem other than cleavage or parting. Habit—The characteristic external crystal shape or form of a mineral. Hardness—How well a gemstone resists scratches and abrasion. Inclusion—A characteristic enclosed within a gemstone or reaching its surface from the interior. Liquid inclusion—Small pocket in a gem that’s filled with fluids and, sometimes, gas bubbles and tiny crystals. Microcrystalline—An aggregate made up of individual crystals visible under magnification. Toughness—How well a gemstone resists breaking and chipping. Trace elements—Atoms in a gem that aren’t part of its essential chemical composition. Twinning—Change in a gem’s crystal direction during or after growth. Twinning plane—Location of a change in crystal growth direction. Two-phase inclusion—A hollow cavity in a gem, usually filled with a liquid and a gas. Unit cell—The smallest group of atoms with both the characteristic chemical composition and crystal structure of a mineral. 135