Varieties of Light
Bill Long 11/03/04
A Plethora of Terms
First it started with fluorescene. Then I went on to phosphoresence. Then, I kept running into different words that try to capture the play of light that emanates from a gemstone, and I was soon overwhelmed. I don't hope to sort out everything in this and the next mini-essay, but I want to introduce several terms, many of which seem to overlap, that try to capture the inexpressible brilliance or murky sheen of a gemstone. Some of these terms can then be cut loose from their association with gems and applied with profit to humanistic discourse.
So, we have some of the following. A stone may be chatoyant or it may display asterism. It can be fluorescent, phosphorescent, triboluminescent or thermoluminescent. Its sheen can be nacreous or a schiller or it can possess adularescence, labradorescence, opalescence, iridiscence or a(d)venturescence. Some of these terms obviously derive from a stone in which they are manifest, and reference will be made to that. There is also a good deal of overlap among terms, and much imprecision, but, like terms derived from ancient property law (sell, lease, convey, demise, etc.), they can be piled up on each other in order fully to describe the mineral under consideration.
Why not list some of them one at a time?
1. Fluorescence. A form of luminescence in which minerals glow when subjected to ultraviolet light. Easy concept.
2. Phosphorescene. Luminescence that seems to come from within the stone when light is trained on it, but the stone continues to emit a shine after the light has been taken away. It is as if energy has been communicated to the stone and the stone "stores it up" as it emits it. This is "glowing in the dark."
3. Triboluminescence. The characteristic of giving off yellow or orange flashes when the mineral strikes against something. This is not very prevalent, but at least we are learning another word!
4. Thermoluminescence. Giving off light when heat is applied to the stone.
5. Chatoyancy. This is a "cat's eye" effect, a slit of light that cuts across the darker surface of a stone. The stone andalusite shows this effect most prominently.
6. Asterism. Derived from the Greek work for "star," asterism is a "star-shaped" or series of pointed lighted dots within the stone, like a series of stars against the dark night sky. The effect is caused by minute acicular (needle-shaped) crystals that are inclusions within the host rock. When all these minute crystals absorb light, the combined effect is to diffuse the light into sparkling asters.
Now, to the Essence of the "Escences"
The most general term for a rainbow-like sheen in a mineral is iridiscence. One dictionary defines iridiscent as "lustrously versicolor," and it can be present in certain minerals, glass, fabrics and even birds and insects. But the word labradorescence seems to be making a bid for as broad a significance as iridiscence. Though labradorescence was associated with the rich, but limited, palatte of colors emitted by polished labradorite, the term has also been extended in its use to describe the colors of spectrolite, which was first thought to be labradorite. Because spectrolite, discovered only in 1940, shows colors of the entire spectrum, and because the play of its colors can be described as labradorescence, it has given iridescence a run for its money in the last few decades.
Then, there is opalescence. It, too, originally was derived from the type of sheen emitted from one mineral, an opal, but then became associated with any milky or pearly emission of light from deep within a mineral. The OED describes it as "milky iridiscence." The first usage of the term shows some of its ambiguity: "Some minerals, when held in particular directions, reflect from single spots in their interior a colored shining lustre, and this is what is understood by opalescence." And, from the approximately the same year (ca. 1815): "Opalescent--transmitting variously colored light combined with a milky cloudiness, as in the siliceous stone called opal." Ruskin could refer to an "opalescent twilight" that characterizes the light of Titian's painting. However, a quotation from 1868 really confuses things --"The beautiful labradorite, or opalescent feldspar."
Maybe it was the imprecision of a comment like this that led the 1911 Britannica Encyclopedia to coin the word labradorescence. Ah, an early example of specialization..this time not by creating the psychology department out of the education department, or sociology out of history, but of labradorescence out of opalescence.
I will conclude these thoughts in the next (brief) mini-essay.
Copyright © 2004-2010 William R. Long