Faking the Grail

Identifying Gems

Lustre

A stone's lustre is its sparkle or sheen. The lustre depends on the nature of the stone's surface reflectivity. Types of lustre include:

adamantine (also called brilliant or diamond-like. Includes zircon and demantoid garnet)
metallic (also known as splendent)
resinous (like amber)
pearly (with an iridescent reflectivity, like opal and abalone)
silky (with a fibrous structure, like hawk's eye and tiger's eye)
vitreous (also known as glassy, like peridot, rock crystal and obsidian)
waxy (like turquoise)

A pearl's lustre is derived from its nacre (mother of pearl).

Refraction

When light enters a medium with a different optical density like water (or a gemstone), the light is bent at an angle and also changes its speed. The refractive index is a measure of how light is refracted by crystals. In doubly-refractive gemstones, the light is split into two light rays when it enters the stone and the rays travel along different paths.

Cleavage and Fracture

Crystals are divided into seven crystal systems, according to their optical properties, plane of symmetry, axis of symmetry, center of symmetry and crystallographic axis. The seven crystal systems are: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic.

Some common crystal habits include:

prismatic (elongated with parallel sides, like emerald and tourmaline)
tabular, (short and flat like morganite)
ocatahedral (eight faces, like diamond, spinel, zircon and lodestone)
dodecahedral (twelve faces, like garnet).

Cleavage is the splitting of gems and minerals along one of the planes related to the stone's structure. Cleavage is considered perfect if the stone parts and produces perfect smooth planes, which is very important in gem-cutting. When a gemstone breaks along a surface that is not related to its internal atomic structure, it is said to fracture. Fracture is the way a stone breaks. Crystalline minerals have cleavage and fracture, whereas amorphous or massive stones only fracture.

A conchoidal fracture is produced when cryptocrystalline stones like flint, obsidian and chalcedony, are broken by an impact of some kind. These fractures result in a broken, curved surface that resembles the rippling, gradual curves of a mussel shell, the Greek word for which conchoid derives. Conchoidal fractures also occur in amorphous solids like glass.

Hardness and Toughness

Diamond, corundum, chrysoberyl and spinel gem stones are hard and tough. Topaz gems are hard but extremely brittle. Garnets are tough, zircons less so, but heat-treated zircons (jargoons) tend to be brittle. So are beryls and tourmalines.

Crystalline quartz is quite tough but not that hard and will fracture like glass if subjected to sudden changes in temperature. Jadeite and nephrite are incredibly tough. Turquoise is tough but soft and porous enough to absorb grease and oil.

Demantoid garnets and peridots are quite soft. Moonstones are soft and brittle and so are opals. Lapis Lazuli is soft enough to cut with tools made of bronze and iron.

Anything softer than lapis is not really durable enough to be considered a gem stone. The hardness of natural and man made glass depends on its composition and varies enormously. Crystal glass, which contailns lead, is the softest.

Moh's Scale

Moh's Scale lists the gemstones in order of hardness and acts as a guide to which stone will cut another. Only diamond cuts diamond, but diamond or carborundum will cut everything else; corundum will cut topaz and beryl; quartz will cut peridot and opal, and so on.

10 diamond
9.5 carborundum
9.0 corundum
8.5 chrysoberyl
8.o true topaz and spinel
7.5 beryl, zircon, almandine garnet
7.25 pyrope garnet, hessonite garnet and tourmaline
7.0 quartz and jadeite
6.5 peridot, demantoid garnet and nephrite
6.0 opal, moonstone and turquoise
5.0 lapis lazuli
4.0 azurite, malachite
3.0 alabaster

NB: The difference in hardness between diamond and corundum is greater than the difference between corundum and alabaster.