Tuesday, July 1, 2008

Transparency and Luster

The two light-reaction phenomena of transparency and luster can almost be thought of as direct opposites. The transparency of a gem is a description of the ease with which light travels through it, while luster is a description of the way in which light is reflected back from it. Transparency is an interesting phenomenon because, except for the evidence of our senses, it seems impossible that anything can pass through a solid substance. And then there is the further problem of explaining how some solids will let the light through and others won't. The fact is that light doesn't go through anything. What happens is that light hits the atoms at the surfaces of these solids and, by its own energy, starts them vibrating sympathetically. These vibrations are passed through the structure from atom to atom. If the atoms are properly aligned, the vibrations will move as in a row of falling dominoes. They are then ejected at the other side in the identical form in which they entered. The same goes for vintage style engagement rings.

The light does not trickle slowly through spaces between the atoms as though finding its way through a maze. The rate of travel of the vibrations is close to 670 million miles an hour, which makes its passage seem almost instantaneous.
Luster, being a reflective effect from the surface of the stone, depends on the quality and quantity of the light thrown back. This, in turn, depends on how the stone tends to reflect rather than refract and also on how well the stone can be polished. Both in turn result from the kind of internal structure it has. Most gemstones reflect like ordinary glass and their luster is described as glassy or vitreous. Some, such as zircon and garnet, have a high luster called "adamantine," or diamond -like. A convenient descriptive classification of the kinds of luster might also include resinous (or greasy), pearly, and silky.

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