When someone wants to buy a gemstone or piece of gem-set jewellery, common questions include:
• Is it a genuine stone?
• Has it undergone any treatments?
• What is its quality?
• Is the price fair?
Proper answers to such questions are crucial for creating consumer confidence. This article addresses one of those questions, that of defining quality. The prices of gems do not always reflect the underlying quality because prices are influenced by many factors completely apart from objective quality. For example, a piece might be priced high if it were bought at a time when the market price was higher; similarly a gem might be priced lower if the seller is in need of money. Thus prices do not necessarily reflect quality.
That said, the gem trade has long used certain criteria for evaluating quality. In order to refine these criteria to improve consumer confidence, the Gem and Jewelry Institute of Thailand (GIT) has conducted in-depth research on the subject, specifically on gem corundum varieties. The objective is to create standards that provide an accurate, reliable measure for coloured stones’ quality.
Methodology
Both diamonds and coloured stones are traditionally graded according to four important quality factors (4Cs): colour, clarity, cut and carat weight. But the impact of each factor on pricing differs from one variety to another. For example, the clarity of yellow sapphire is, on the average, higher than blue sapphire, and blue sapphire is typically of higher clarity than ruby. Hence, clarity defects in ruby have less of an impact on price than the same defects in blue sapphire.
GIT's quality assessment system for gem corundum has been developed and continually refined through consultations with both gem traders and gemologists (Janthayot et al., 2001, 2003a, 2003b, 2003c, 2004, 2007; Siripant 1999; Siripant et al., 2001 and 2003). The institute has established coloured stone master sets for eight varieties of gem corundum, ie, ruby, blue sapphire, padparadscha, yellow sapphire, pink sapphire, purple sapphire, orange sapphire and green sapphire. All master stones are oval shaped and of at least 0.50 carat each.
GIT’s standards are based on the Munsell system. Common Thai colour names were first converted into Munsell colour codes, and then modified so that they can be better understood by relying on the colour code charts of the ISCC-NBS (Inter-Society Color Council-National Bureau of Standards) system.
Quality factors of ruby and sapphire were determined by polling gem dealers and gemologists, both at GIT and at international gem shows. The resulting data was then used as a foundation to establish the eight primary sets of master stones.
Colour grading methodology is as follows. Stones must be placed 25 cm from the standard light source, which consists of a Gretag Macbeth 5,000° Kelvin lamp with an intensity of 1,200 lux. This lamp is designed to simulate north daylight. Stones are visually graded in the face-up position at a distance of 30 cm. Hue, tone, and saturation of the stones are considered; dispersion and scintillation are excluded. Overall stone quality was also taken into account, with stones chosen to be used in the standard sets needing to be similar in clarity, size and cutting style.
Notable colours: pigeon’s blood and royal
blue Among the 4C's factors, colour has the greatest impact on price, particularly at the high end of the price scale.
In former times, the finest colours of ruby and sapphire were referred to by ill-defined terms such as pigeon's blood red, royal blue and cornflower blue.
‘Pigeon's blood’ (ko-twe in Burmese) has been variously compared to drops of blood from the nose of a freshly-slain pigeon to the colour of the eye of a white pigeon. The famous British gemologist, J F Halford-Watkins, who lived many years in Mogok, described it as a rich crimson without any trace of blue overtones (Hughes, 1997).
Typical terms for sapphire include ‘royal blue’ and ‘cornflower blue’
Corundum colour master sets: pigeon’s blood ruby and royal
blue sapphire Because of the ambiguity of these colour terms, GIT set out to create master sets to define them (Janthayot et al., 2001, 2003a and 2007). These master sets were created by consultations with both gem dealers and gemologists. Each master set comprises eight master stones in a range of vivid red or blue with intense colour and medium to dark tone.
These master stone sets for ‘pigeon’s blood’ and ‘royal blue’ colours illustrate, and identify common colours used in the gem trade. By standardising such colour definitions, it is hoped that this will help consumer confidence.
Other master sets
GIT has also conducted research on clarity and cut grading standards for ruby and blue sapphire.
‘Cut’ is defined by a gemstone’s proportions and finish. Proportion refers to the dimensions and overall symmetry of a gemstone, while finish describes the precision and neatness of the facets and polishing quality. Symmetrical face-up and profile symmetry, combined with crown-to-pavilion proportion rations of about 1:3 to 1:4 lead to higher cutting grades.
Unfortunately, due to very high value of such stones, cutters are forced to maximise weight retention. Hence, very few stones possess ideal proportions.
Clarity is another factor that influences the value of ruby and sapphire. Clarity is determined based on the size, number, position and nature of inclusions. Ruby is rarely loupe-clean or even eye-clean. In contrast, sapphire generally features fewer inclusions, and thus loupe-clean or eye-clean stones are the standard in the trade.
Conclusion
GIT’s ruby and sapphire quality evaluation system is intended to be used as a general standard in the gem and jewellery industry. The ultimate goal is to use these standards as tools for promoting the coloured stone trade and building consumer confidence.
This article was written by Thanong Leelawatanasuk, Wilawan Atichat, Visut Pisutha-Arnond and Pornsawat Wathanakul and GIT. Acknowledgements and references are listed on P.51 of InColor Spring 2014.
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