"Cut", "fire" and "luster" of a gem. What is it? (Part 2 of 5)

Saturday, May 22, 2010

Beauty is another characteristic that affects a gem's price. You can see, that your favourite gem is beautiful. How can you describe it to others?

Beauty is a color, whether it is invigorating and warm color of a ruby or mysterious color of an emerald. A shape and a cut matters a lot too. They can show the beauty of a stone or kill it.

Only a professional gemologist can see if there is an interesting optical effect in the rough gem and choose the cut, that should be individual. Transparent stones mostly have faceted cuts, but if there is a "star" or a "cat's eye" effect then the stone must be cut as a cabochon.

How really beautiful is your stone? You need to know what to look at, so look at its lustre, brilliance and "fire".

Lustre is an optical effect produced by the light reflected from the surface of a mineral. Rays of light partially pass through a stone and they are partially reflected too. So, the brilliance depends on the internal nature of the stone on one hand and on the surface and the quality of its polish on another hand.

Look closely at your stone. How well is it polished? The surface should be smooth and shiny. If it is, you will be able to see the most brilliance and lustre that your stone can show.

Most transparent and translucent gems have brilliant (adamantine) and glass (vitreous) lustre. What lustre does your stone have? Does it have the same lustre as glass or is it much brighter, like a diamond?

Vitreous lustre is a characteristic of a quartz, sapphire or topaz.
Adamantine lustre is a characteristic of a diamond, zircon and demantoid (green garnet).
Resinous lustre is a lustre like that of a resin (examples: some garnets and amber).
Pearls and moonstone has pearly lustre.
Satin-spar has silky lustre.

If there is "fire" in a stone then it adds more beauty to it. Move your stone under the light. Do you see the sparkles of rainbow? Fire is especially important for the colourless stones. Usually the gems judged by the combination of lustre and "fire". A cut is chosen individually for each gem in order to enhance it. The most brilliant stones that have the best combination of both: diamonds, zircons, garnets almandine, rubies, sapphires, spinels and topazes.

Maria Roudakova
A jeweller from Switzerland


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