Loughborough 2009 (Part Two)

The first workshop I attended, titled ‘A Practical Guide to Jadeite: Identification & Value”, was hosted by Rosamond Clayton, a Hatton Garden based jewellery valuer and acknowledged specialist in jadeite values. Attendees were given samples of jadeite imitations (e.g. serpentine, glass, Maw-Sit-Sit) and treatments (stained and polymer impregnated samples). Illustrations of imitations, antiques jadeites, the history of importation into China from Burma, the so-called 83-Jade (the low quality material treated to produced B-Jade (acid soaked and polymer impregnated) and the A-, B-, B+C, and C-jade were shown. Finally we were asked to assign values to various samples of natural, treated and imitation jadeites.

Barbara Leal, of the independent jewellery valuers, Jewellery Evaluations & Mediation Services, offered a main presentation on colour assessment of gems titled ‘Colour Speak’. Barbara spoke of how the colour of a gem is expressed sometimes in terms related to Nature, such as Cornflower blue (sapphires), Forest green and Pigeon’s Blood red (ruby) and illustrated the impression of these terms. She emphasised the value of a coloured gem is determined chiefly by its colour (she quoted colour as 70% of value, clarity 15% and shape and cutting precision as 15%) and reviewed the Munsell colour communication system utilising the colour “co-ordinates” of hue, tone and saturation forming a three-dimensional arrangement of colour and how this system has been adapted to describe and communicate the colour of gems. There are seven principle hues or colours; red, orange, yellow, green, blue, violet and purple and 24 modified colours, such as red-orange and yellowish green, totally 31 hues, forming the first dimension of hue. As an example a garnet may be orange but have a modifying red colour, being reddish orange.

The second co-ordinate, tone, is the lightness to darkness of a colour, graduated to nine numerical values, with greys between the extremes of white (0) and black (10). The tone of most gems would be judged between 2 and 8. As an example the tone of our orange garnet may be described as medium light (4).

The third co-ordinate is the dull to vivid purity of colour called saturation. The dull saturation of red, orange and yellow gems (warm colours) will display brownish purity whereas greyish dullness will be observed in the cool colours of green and blue gems, The extent of saturation is graduated from 1 to 6, vivid being 6. For example an orange garnet may be judged to have a very slightly brownish (3) saturation

The colour of gem judged to possess a hue of reddish orange (rO), having a medium light tone (4) and a very slightly brownish saturation (3) can be expressed as ‘medium light, very slightly, reddish orange’ or as ‘rO 4/3’.