Gemstone Colour Determination

Abstract

The article describes the most important component of gemstones: their colour. The subject of the article is determination of the colour of diamonds with the most frequently encountered yellow tones of different saturation, fancy-colour diamonds as well as emeralds, rubies, and sapphires. For non-ambiguous determination of the colour of gemstones and elimination of human factor, the comparison method known in photometry, i.e., comparison of the colour of a given diamond with the colour of a reference sample is used. The article describes methods of determination of the colour of yellow-hue diamonds when assessing them visually, using reference colours: a set of diamonds. Information on colour groups of diamonds of different cuts used in Russia and the system of Gemological Institute of America (GIA) is provided. Fancy colours of diamonds are very variable and, therefore, they have no references such as the ones for yellow-hue diamonds. For evaluation of fancy colours, the Munsell Book of Colour is used. It includes 1600 colour samples on 40 maps each having its own colour hue. The samples are arranged in rows and columns: samples of the same row have the same brightness; samples of the same column have the same saturation. As the samples for determination of fancy colours of diamonds as well as emeralds, rubies, and sapphires plastic plates of different colours are used. The colours of the examined gems are compared to them. The article describes the methods of assessment of fancy colours of diamonds adopted in Russia. Nowadays, a software colour evaluation system is proposed by GIA to be used as an alternative to colour samples. Gemewizard software proposed by Izrael may identify 31 main colour tones. It reproduces each tone in six colour hues. Each hue is divided by six brightness levels. In general, this system allows us to determine 1116 colours of gemstones. The software approach had been already proposed previously but computer screens have different colour rendering, which is still the main hindrance for determination of gem colour by means of software.