Assessing Colored Gemstones, Part 3 - Clarity

In this third part of our JET Team discussion about how fine colored gemstones are assessed, we look at how clarity contributes to the overall gemstone grade. All gemstone images shown are courtesy of Heritage Auctions.

Clarity is the second most important factor in grading colored gemstones, and describes any inclusions (flaws) within or blemishes on the surface of a gemstone. The higher grades are usually given to gemstones that are "eye clean", with no flaws visible to the naked eye. However, flawless colored gemstones are very rare indeed, and will therefore command very high prices.

Thus, unlike diamonds, unless the flaws are very large and/or numerous, or likely to affect the durability of the stone, minor inclusions don't usually affect the price of colored gemstones. Indeed, with some gemstones they are regarded as acceptable - these include sapphires, rubies, garnet and peridot. Star sapphires, for example, aren't star sapphire unless they exhibit needle inclusions!

From left: Very good oval kunzite, 17.31ct Very good oval paraiba tourmaline, 4.15ct Fair black opal cabochon, 7.24ct

Other stones, such as emeralds, beryls and tourmaline are very rarely found in nature without inclusions, and a long-established and industry accepted way of improving the clarity of emeralds, for example, is to fill them with oil or resin to make the inclusions less visible.

Because of the clarity differences between different types of gemstones, the untreated stones themselves are usually classified as Type I, Type II or Type III gemstones:

• Type I coloured gemstones usually grow with no inclusions visible to the naked eye, including aquamarine, morganite, tanzanite and blue topaz
• Type II coloured gemstones usually grow with some minor inclusions visible to the naked eye, including garnet, peridot, ruby and sapphire
• Type III coloured gemstones almost always have inclusions visible to the naked eye, including emerald and tourmaline

From left: Very good pear-shaped citrine, 59.36ctGood round-cut demantoid garnet, 3.67ctVery good oval colour changing alexandrite, 5.36 ct

Opaque gemstones are graded differently again since by their nature they do not transmit light, but they should still reflect light well. they should not have any cracks or fissures, or feature undesirable inclusions. But as always there are exceptions! For example, inclusions such as veining can be acceptable - and sometimes even desirable - in certain types of stone, such as lapis lazuli.