Gemstone Information - Ruby Mill

Colour has characteristics that are described separately as:

Hue - the actual colour seen, i.e. primary colours such as red and blue. When the colour is between two colours, then both colours are mentioned, i.e. yellowish-green or bluish-purple.
Saturation - the amount of colour present. Gemstones are often heat-treated or irradiated to increase the colour saturation.
Tone - the lightness or darkness of the colour present, also described as the Depth of Colour.
Colour Purity - the purity of a colour is influenced by the presence of grey or brown. When grey or brown is absent then the colour is of high purity; often termed as vivid or intense. Colours of high purity can be seen in the light dispersed through a prism or the facets of a white diamond.

Some gemstones show more than one colour, for example:

Parti-coloured stones - show two separate colours in one stone, an example is Ametrine.
Colour change stones - are one colour in natural light and another in artificial light, an example is Alexandrite.
Multicoloured stones - show at least three colours, an example is Multicoloured Opal.

The amount and quality of light reflecting from the surface of a gemstone is known as its lustre.
For example, vitreous is a "glass-like lustre" and adamantine is the 'very high lustre' seen on the surface of a polished diamond. The lustre of gemstones are also described in terms that are easy to relate to, i.e. metallic, resinous, pearly, silky, greasy, waxy and dull.
It is usually found (with exceptions) that the harder the gemstone material the brighter the lustre.
The quality of the surface polish is important, as a poor polish will display a poor lustre.

Dispersion is the splitting of white light into its spectral colours, as with light transmitted through a prism. It is also called fire when describing the dispersion seen in gemstones; diamond is a good example with a high dispersion value. Diamonds may be cut to increase dispersion at the sacrifice of brilliance or vice versa as described above in the brilliance description.
The fire in opal is caused by the dispersion of light through its unique hydrous silica structure.
The amount of dispersion in gemstones can be given in figures, but is generally described simply as low, medium and high.

Dichroism - is a gemstone property, where two colours or shades of colour are seen when a gemstone is viewed from different directions.
Trichroism - is a gemstone property, where three colours or shades of colour are seen when a gemstone is viewed from different directions.

The ability of a crystalline gem material to split or crack along a flat plane within its crystalline structure. Cleavage can be perfect, imperfect, poor or it does not occur.

The Mohs' scale of hardness determines which stones are harder than others. Harder stones have the ability to scratch softer stones and are positioned higher on the scale. The scale is a 'comparison scale' only and does not determine absolute hardness. Diamond is the hardest at 10 on the scale and talc is the softest at 1.

The resistance to damage or critical fracture when confronted by a hard physical knock or pressure. Cleavage is a factor when considering toughness.

The durability of a stone takes into account its hardness, toughness and ability to resist chemical erosion. All factors must be considered; a hard stone for example may be brittle and suffer damage to the facet edges.

Some gemstones are susceptible to sudden changes in temperature, termed thermal shock. This may cause crazing (surface cracking) or serious fractures. Immersing hands in hot water whilst wearing rings set with gemstones or moving outside from warm to freezing conditions are examples of sudden or severe temperature change.

A gemstone is called a variety when it belongs to a species, e.g. emerald and aquamarine are both varieties of the species beryl. Varieties must have significant differences to be called varieties, such as a colour variation or special optical effect.

To be a species there must be at least two varieties, eg. corundum is a species with the varieties sapphire and ruby. A species can exist on it's own or may belong to a mineral group.

A mineral group consists of members. In some mineral groups each member may be a species with a number of varieties. Whereas other mineral groups may have members as species and also single members.