Tuesday, February 07, 2012

Light goes in and comes back out

kw: minerals, gemstones

OK, time to drop the other shoe. My last post was about the importance of hardness in the beauty of a gemstone. Here we can take a quick look at brilliance and fire, as they are called. Brilliance is a somewhat subjective measure of the amount of light that can enter the stone and get back out again. The total light that meets the eye, though, is a combination of the white light that comes back white, and the portion that is refracted into a colored beam, and only one color reaches the eye. The refracted part is the Fire. It partially competes with brilliance, because if red light is refracted toward the eye, all the other colors went somewhere else, so the total light is less.

This assumes that the stone is optimally cut, a subject I can't cover in detail, partly because there are still epic disagreements over what constitutes the "right" cut for a particular material. Just in general, the higher the refractive index is, the smaller the angle of total internal reflection, so the shallower a stone can be and still reflect the maximum amount of light back out to add to the brilliance. Thus, each stone on the table below will be cut to a different set of angles for optimum brilliance. The level of fire is set by the dispersion, which is the difference between the exact refractive indexes for blue versus red light.

This blogging tool handles tables poorly, so I used a picture of a table produced elsewhere. Moissanite, an artificial material, has both a higher refractive index than diamond and a much higher dispersion, so people who are familiar with the "look" of a diamond can see right away that a Moissanite stone looks overdone.

Cubic zirconia, however, is optically very similar to diamond, so it is quite hard for a non-expert to distinguish without a specialized instrument.

Although Paste has a much lower refractive index than all these, its dispersion is very similar to diamond, which makes it a good stand-in. A well-cut paste stone is quite stunning, and only when one is placed right next to a diamond of similar size can most of us tell there is a difference.

Quartz is included primarily for comparison. Colored varieties of quartz were once useful stand-ins for corundum and clear emeralds, but the availability of low-cost synthetic crystals has just about eliminated its use. Bottom line: if you want a diamond, by all means get one, but there are more choices for lovely, affordable stones than ever before in history.

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