When crystals form, their atoms and molecules are locked together in period arrays, much like three-dimensional wallpaper patterns. These arrays have symmetry of various types that can be described and categorized into one of six major crystal systems: isometric, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic.

Gemstone identification is like detective work. You gather clues and eliminate suspects until you’re left with clues that lead to only one suspect … or gemstone. Since few gems share the same refractive index or RI, testing an unknown gem’s RI is a good way to start an investigation. The gem refractometer is the most straightforward and accurate tool for refractive index testing.

In gemology, hardness means “the ability to resist scratching”. A diamond will scratch a ruby, therefore it is harder.

Specific gravities are usually measured with heavy liquids. A liquid is prepared, such as a mixture of bromoform and toluene, to have a specific density value. An unknown material dropped into the liquid may sink, float, or remain suspended in one place within the liquid. If the material sinks, it is denser than the liquid, and if it floats it is less dense. If it remains at one level it has the same density as the liquid.

Gems that polarize light and split it into two (or three) different directions are said to be doubly refractive. Birefringence is a measure of a gem’s double refraction and it’s one of the principal ways gemologists can identify gems.