Gem ID Tip: Lustre
Today, let's talk about a gemstone trait that’s commonly overlooked, but which (when you get the knack!) can be a very useful tool in gemstone identification.
The trait? It’s lustre.
Lustre is the quality and quantity of light returned from the surface of a gemstone. The key word here is surface.
Take a look at any polished surface you have to hand, and angle it at the light, as you move it do you see white light being returned from the surface? Think of glare from a window on a sunny day. There’s your lustre.
And the really cool thing about lustre in gemmology is that gemstones have different and unique lustre on polished surfaces. Which can really help you whittle down gemstones even at a glance, once you get the hang of it, simply by identifying its lustre.
Lustres run from the best, whitest return of light - adamantine - and run down, sub-adamantine, bright vitreous, vitreous (as in, the same as glass), dull vitreous, resinous, waxy, pearly and metallic.
Let’s focus on Adamantine through to vitreous, because this is where honing your eye to spot lustre becomes useful.
Adamantine = Diamond. Adamantine, aka unparalleled, is a lustre reserved for diamonds alone. It’s bright and mirror-like, and unmatched in diamond simulates such as cubic zirconia, white sapphire, rock crystal or synthetic moissanite (though admittedly, this one comes very close). If you have diamonds of any size, compare them to any other gemstone, and you’ll see it.
Sub-Adamantine = Zircon & Synthetic Moissanite: Zircon has a wonderful lustre (it’s a shame it is prone to chipping due to being brittle), as does synthetic moissanite, which is part of why it makes such an impressive diamond simulate
Bright Vitreous = Sapphire & Ruby: Certainly a part of their beauty, these two gemstones have a strong lustre which can tell them apart from glass or in the case of Ruby, garnets.
Vitreous = Literally meaning glasslike, vitreous lustres return about as much white light as glass, and include your beryls (Emerald, aquamarine, morganite), topaz and many other gemstones.
Dull vitreous / greasy = peridot: Not a flattering description, but while many gemstones that are less lustrous than glass are opauque or translucent (Opal, turquoise, coral, pearl), not all! Peridot is often described as having a greasy lustre.
I hope this is interesting gem aficionados! Now of course we cannot identify a gemstone upon lustre alone, and lustre won’t help us determine if a gem was naturally formed or made to be chemically identical in a lab, but it is a fun trick to practise, and when you’re good at it, you’ll be a lot smarter at spotting diamonds from their simulates, and natural gemstones from paste.
Let me know how you get on!
