Colour & purity
In terms of pure chemical composition, most coloured gemstones would be colourless. They obtain their colours by their content of a small quantity of trace elements. Sapphires obtain their colours through iron and titan, rubies and emeralds through chrome.
Above all, colour and purity determine the value of a stone. A gemmologist judges every stone according to its primary colour, uniformity and the intensity of its colour. The fewer inclusions a stone has, the more valuable it is. However, inclusions can also be interesting: very often they confirm the authenticity and the origin of the stone.