Your first steps
into the world of gems.

One mineral.
Several gemstone names.

A gem name is not always the name of a mineral. Ruby and sapphire are both corundum. Emerald, aquamarine and morganite are varieties of beryl. Different colors and characteristics can give varieties of the same mineral different gem names.

In a crystal, atoms or other building units form an ordered structure. Classification depends on internal symmetry, not just visible shape. In the seven-system convention, familiar gem minerals can be grouped as follows.

Color depends on its encounter with light.

A gem's color relates to its constituent elements, crystal structure and inclusions. Compare hue (the kind of color), tone (lightness or darkness) and saturation (intensity) separately. Keep the lighting and background consistent when comparing stones.

Pleochroism

The appearance of different colors or color strengths when a crystal is viewed in different directions. Examples include tourmaline and tanzanite.

Color change

A change in apparent color under different light sources. Alexandrite is a well-known example.

Play-of-color

Flashes of color that change with viewing angle. In precious opal, this involves diffraction by its microscopic structure.

Cat's-eye and star effects

Bands or stars of light produced by reflections associated with oriented inclusions. The gem is polished as a cabochon to display the effect.

Colors in photographs and on screens depend on lighting, photography and display settings. Color alone cannot establish a mineral's identity, treatment or geographic origin.

Scratch resistance
and resistance to breakage.

The Mohs scale compares how minerals resist scratching, ranked from 1 to 10. The steps are unequal: a hardness of 10 does not mean twice the hardness of 5. Scratching a finished gemstone can damage it; do not test a polished gem this way yourself.

Mohs reference minerals, equally spaced here to show their ranking.

Hardness

Resistance to scratches and abrasion. Diamond is 10, corundum 9 and quartz 7 on the Mohs scale.

Toughness

Resistance to chipping and breaking. Fine aggregate structures can give jade considerable toughness.

Cleavage

A tendency to split along particular crystal directions. Even hard gems such as diamond and topaz can chip when struck.

Stability

Resistance to heat, light and chemicals. Color fading and changes to treatment materials can affect some gems.

Origin describes where a gem formed.

Gems form in geological settings around the world. The representative sources in this dictionary are examples of known localities. They are not current production rankings or statements about the origin of an individual stone.

Origin work examines inclusions, composition and spectral properties and compares them with reference stones from known localities. Different sources can share similar features, so an origin conclusion is not always possible. A locality name alone does not determine quality.

Choose a shape. Bring out the light.

Round, oval, pear and cushion describe a gem's outline when viewed from above. Brilliant and step describe the arrangement of its polished facets. Gems with the same oval outline can have different facet arrangements.

These are schematic cutting-style diagrams. Cutters consider rough shape, color distribution, pleochroism and inclusions. Pearls are normally used without cutting.

Confirm identity and treatment separately.

Natural gem

A gemstone formed in nature. Natural does not automatically mean untreated.

Synthetic gem

A gem grown by people, with essentially the same chemical composition and crystal structure as its natural counterpart.

Imitation

A different material used to resemble a gem, such as glass used to imitate ruby.

Treatment

A process used to change color or apparent clarity, including heating, irradiation, impregnation, filling or dyeing. Care requirements depend on the treatment.

Identification examines gem type, natural or synthetic origin, and detectable treatments. Its purpose differs from quality grading or estimating a selling price.