03Jan Basic Gemstone Terminology

If you are new to the world of gemstones, here are a few terms and definitions that you might find useful.

Carat (ct) - a measure of weight of gemstones. One carat is a fifth of a gramme (0.2g).

Inclusion - an internal feature of a gemstone, which could be a solid, liquid or gas that the crystal enclosed as it formed or fractures, cracks and cleavages that occur during or after the formation. Many inclusions are viewed as defects, but some can enhance the beauty of a gemstone.

Lustre - the way in which light is reflected from the surface of a gemstone. Most gems have a vitreous or glassy lustre, although diamonds can have an adamantine lustre.

Dispersion / Fire - the splitting of white light into its component parts, giving a rainbow effect within a gemstone.

Refractive Index (RI) - the amount of refraction within a gemstone, which is a measurement of how much a beam of light bends as it enters the dense gemstone material from the air. This value can be useful when identifying a gemstone.

Hardness - a value from 1 to 10 which indicates a gemstones resistance to scratching as defined by the Mohs Hardness Scale.

Specific Gravity (SG) - a measurement of the density of a gemstone. This value is the same for both rough and faceted gem material and can help identify the stone.

Facet - a flat surface of a cut and polished gemstone, ideally positioned to maximise the reflection and refraction of the gem and its visual appeal. The main facets of a gemstone are usually the table, crown, girdle, pavilion and culet.

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