SPECIFIC GRAVITY

Gem materials vary greatly in density -- amber may float in salt water (density near that of water), while hematite is more than five times the density of water. This is why two different gemstones may have the same size but different weights and vice versa -- a one carat round brilliant diamond of typical proportions will be approximately 6.5 mm in diameter, while a round brilliant ruby of the same size (6.5 mm in diameter) and proportions will weigh approximately 1.55 carats. Generally, gemologists refer to specific gravity, or relative density -- the ratio of the density of a gemstone relative to that of water. The specific gravity (SG) of a gem is an indication of its density. It is calculated by comparing the stone's weight with the weight of an equal volume of water. Gem materials vary greatly in density -- amber may float in salt water (density near that of water), while hematite is more than five times the density of water. This is why two different gemstones may have the same size but different weights and vice versa -- a one carat round brilliant diamond of typical proportions will be approximately 6.5 mm in diameter, while a round brilliant ruby of the same size (6.5 mm in diameter) and proportions will weigh approximately 1.55 carats. Generally, gemologists refer to specific gravity, or relative density -- the ratio of the density of a gemstone relative to that of water.The greater a stone's specific gravity, the heavier it will feel. For example, a small cube of pyrite, with an SG of 5.2, will feel heavier than a larger piece of fluorite with an SG of 3.18; and a ruby (SG 4.00) will feel heavier than an emerald (SG 2.71) of similar size.

RELATIVE WEIGHTS
The smaller piece of pyrite (SG 5.2) feels heavier than the fluorite (SG 3.18), because it is more dense.

CLEAVAGE AND FRACTURE

Cleavage is the splitting of gems and minerals along one of the planes related to the stone's structure. Crystalline minerals have cleavage and fracture, whereas amorphous or massive stones only fracture. Gemstones may break in two ways: they either cleave or they fracture. Which way they break depends on the internal atomic structure of the stone. Gems that cleave tend to break along planes of weak atomic bonding (cleavage planes). These planes are usually parallel, perpen­dicular, or diagonal to the crystal  faces (as both planes and faces are directly related to the stone's atomic structure). A gemstone may have one or more directions of cleavage, which may be defined as perfect (almost perfectly smooth), distinct, or indistinct (examples are shown right). Gems with perfect cleavage include diamond, fluorite, spodumene, topaz, and calcite. When a gemstone breaks along a surface that is not related to its internal atomic structure, it is said to fracture. Fracture surfaces are generally uneven, and each type has its own descriptive name, shown in the ex­amples below and right.

 
BARYTE
Perfect Cleavage Fragile baryte has three directions of easy cleavage, giving smooth surface.

ALBITE
Distinct Cleavage Although not perfectly smooth, albite's cleavage surfaces can be clearly seen.

AQUAMARINE
Indistinct Cleavage Cleavage direction is indistinct in aquamarine

Uneven Fracture
An uneven fracture surface is typical of fine-grained or massive gems like dumortierite.

OBSIDIAN
Conchoidal Fracture  The type most commonly found in gemstones, the name refers to the shell-like fracture surface.

GOLD
Hackly Fracture
Rough, uneven fracture surface, seen on the right of this gold specimen

Splintery Fracture Interlocking texture causes splintery fracture.