Precious Stones Clarity
During the last decades, clarity has gradually become more important in the assessment of gemstones. Earlier precious stones were primarily about colour and clarity was considered by many a distant second. However, with the advent of diamond grading, an increasing emphasis is nowadays also being placed on the cleanliness of coloured stones.
A gem’s clarity should always be judged initially with one’s unaided eye using no magnification whatsoever. A loupe helps in reaffirming certain observations about a particular stone and identifying any possible durability issues and/or potential treatments.
Coloured gemstones are broadly classed as being either free from inclusions or as being minor, moderately, heavily and lastly excessively included. This scale is somewhat reminiscent of diamond grading schemes.
Total eye-cleanliness, excepting Emeralds where very minor inclusions are permissible, is the benchmark of true ‘gem’ quality material, the finest of the fine in coloured stones. But gem quality is more than just about clarity, it also encompasses both a supreme cut and superior colour. And many bright, rich colours are often caused by rare, volatile elements which are introduced by hydrous fluids during crystal growth making these gems particularly prone to inclusions. Thus clean stones often tend not to have the richest colour and if they do they should be considered true rarities. Rather minor or inconspicuous inclusions are the norm, even in fine qualities and inclusions as such are only to be considered flaws when they affect the beauty or durability of a stone.
Clarity also encompasses the general ‘lucid’ appearance of a gem known as transparency.
This overall degree of clarity can be split into varying levels. These reach from transparent (and possessing the finest ‘crystal’) through semi-transparent, translucent down to semi-opaque and lastly opaque.
Most fine faceted stones should be transparent. Some cabbed phenomenal gems such as star rubies tend to be more semitransparent to translucent. They actually necessitate inclusions such as fair amounts of rutile ‘silk’ to display their effect to the best.
‘Crystal’ as the smaller subset of transparency only becomes truly relevant in finer qualities where an exemplary crystal separates the best stones from the rest. It may, for example, be described as either sleepy or crisp.
Inclusions are further helpful in determining particular origins should basic crystal chemistry be inconclusive. Or according to the GIA small and few but characteristic inclusions provide identifiable characteristics with regards to source origin and hence provide desirability and add value to a gem. ‘Silk’ leading to the increased sleepy refraction and rich velvety colour typical of Kashmir sapphires is a notable example. Other gemstones even necessitate certain inclusions to reach the highest collectable status, such as Russian Demantoid garnet showcasing picture-book chrysotile ‘horsehair’ inclusions.
Despite the well known gemological laboratories including for example SSEF, Guebelin having their own clarity grading standards, all labs broadly adhere to the earlier mentioned broad clarity divisions.
The American Gemological Laboratories (AGL) for example use a scale that ranges from FI or ‘Free of Inclusions’ through LI1 and LI2 ‘lightly included’, ‘Moderately (or noticeably) Included’ MI1 and MI2 and ‘Heavily (obviously) Included’ HI1 and HI2 to ‘Excessively Included’ I, II and III containing prominent inclusions. AGL grades in absolute terms which make relative clarity comparisons easy. Additionally the AGL grades ‘texture’ which covers clarity characteristics that do not strictly fall into aforementioned categories yet affect transparency and ‘life’ of a gem. Generally, stones with a grade up to MI2 are considered suitable for fine jewellery.
Although the GIA uses clarity grading categories similar to AGL, it applies those grades differently according to different gem types. These Types group different gem species or colour varies according to their natural predisposition towards inclusions which is often correlated with their geological origin and growth conditions. Type I stones are usually eye-clean with no visible inclusions being the trade standard (e.g. Aquamarine). Type II gemstones usually feature some minor eye-visible inclusions that are well tolerated within the industry and include the majority of gems such as Sapphires and Spinels. Type III gems include notably Emeralds and are very rarely free of inclusions so even specimens with more prominent inclusions are commonly faceted. The effect any clarity features hold on durability and appearance is evaluated separately by the GIA. GIA grades consider that gemstones should always be compared against a set of standards applicable to a specific gem species. Contrarily AGL grades in terms of absolute clarity standards, irrespective of gem type.
Common inclusions found in precious stones include negative crystals which are essentially voids, various mineral inclusions for example isolated rutile (titanium oxide) needles or fine clouds of needles called ‘silk’, fluid inclusions, ‘Three-Phase’ inclusions consisting of a cavity containing trapped liquid with an exsolved crystal and a gas bubble. The latter inclusion is typical of Colombian emeralds but has also been identified in Afghan material.
Other common inclusions include healed fissures aka ‘fingerprints’ (often seen in rubies and sapphires) with partially resorbed texture, cracks, parting, crystal twinning lamellae and tension halos which are essentially fractures around a crystal caused during formation due to a different expansion coefficient of mineral inclusion.
Lastly, any external damages including scratches, pits, nicks and abrasions should be checked along with assessing clarity. Collectively known as ‘blemishes’ these may become clarity issues once they penetrate the surface.