A combination of hue, saturtaion and tone.
The world we live in is full of light, which can be broken down into seven different wavelengths, with each wavelength relating to one of the seven colours seen in a rainbow: red, orange, yellow, green, blue, indigo and violet. Each of these wavelengths has different lengths: the bluish colours are the shortest, the greenish colours fall in the middle and the reddish colours are the longest.
When light hits an object, depending on the type of material, some colours are absorbed and some are reflected. The reflected wavelengths are what our eyes perceive as the colour of the object.
The different hues, tones and saturations (the three main elements of colour) seen in gemstones are due to the presence of differing elements and often the same gemstone can naturally be found in a multitude of different colours. Take Garnet, Spinel and Tourmaline: these three gemstone families are available in a wide spectrum of colours, and this is due to differing chemical compositions, different physical structures, and sometimes caused by the presence of inclusions.
The depth of colour in a gem, amongst other things, can greatly affect the value of the stone. Generally speaking, the more vivid the colour, the greater the value. Unfortunately, with some gems such as Emeralds and Bixbites, better depths of colour are normally caused by a greater occurrence of inclusions and imperfections. When it comes to Diamonds, valuations relating to colour are very different compared to almost all other gems. A Diamond’s value is usually greater the nearer it is to colourless.
There are many reasons why coloured gemstones have been worn for thousands of years. Folklore, legends, myths, healing benefits, talismans, birthstones and zodiac gems are all exciting topics that relate to gemstones and often act as an inspiration to acquire a certain piece. But for me, the most important motivator for buying any gemstone is its colour. If it’s vivid, if it’s beautiful, if it reminds me of other colours seen in nature, then I simply have to have it!
See also hue in this volume, and tone and saturation in volume II.