The Science of
Gemstone Colour

The Science of Gemstone Colour

The colour of a gemstone comes mainly from how it interacts with light. White light contains many different colours, but a gemstone absorbs some colours and allows others to pass through or reflect back to our eyes. The colours that reach our eyes determine what colour we see.

Tiny amounts of certain elements, called impurities or chromophores, are often responsible for gemstone colour. For example, chromium gives ruby its red colour and helps make emerald green. Iron can produce yellow, green, blue, or brown colours in different gemstones. Copper, manganese, titanium, and other elements can also create beautiful colours.

Colour grading is one of the most critical aspects of gemstone evaluation. For diamonds, the GIA colour scale ranges from D (completely colourless) to Z (light yellow). For coloured gemstones, the evaluation considers hue, tone, and saturation — the three pillars that define a stone's visual appeal and market value.

The crystal structure of a gemstone is also important. The same element can create different colours when it is surrounded by different atoms or is present in a different crystal structure. This is why ruby and emerald can have different colours even though both can contain chromium.

Some gemstones get their appearance from special structures rather than impurities. For example, opal shows flashes of different colours because tiny silica particles interact with and interfere with light. Star ruby and star sapphire can also show a star-shaped effect because of tiny needle like inclusions inside the stone.

Gemstone colour can also change depending on the type of light. Some stones, such as alexandrite, may appear green in daylight but reddish under warm artificial light.

Thus, gemstone colour depends on chemical composition, crystal structure, light, and the way our eyes perceive colour.