Moon Beams to Slide Down

One tends to think of dictionaries as reference books, sources of information, providers of meaning. Here is the definition of the mineral selenite in the Collins dictionary:

"A colourless glassy variety of gypsum. (C17: via Latin from Greek selenites lithos moonstone, from selene moon; so called because it was believed to wax and wane with the moon."

Is this not more of a puzzle than a definition?

'Wax and wane' refer to the changing portion of the illuminated moon. Since the moon influences the tides on the earth, we also say that these wax and wane meaning they become more powerful (Spring tides) or less powerful (Neap tides).

But how does a crystal wax and wane? Is it some physical properties of the crystal that become stronger and weaker?

Or is the crystal, which can form long glassy wands, interpreted as a solidified moon beam which must therefore change its properties as the moon becomes brighter or dimmer?

This is what Giambattista della Porta (1535-1615) has to say in his book "Natural Magick":

"...The stone Selenites, (as much as to say the moonbeam) called by others Aphroselinon, contains in it the image of the moon, and shows waxing and waning of it every day in the same image..."

We are evidently witnessing a natural property of the rock to which some phenomenon has been attached by a 'susceptible' person. I think scientists have to be rather cautious of the witness of 'susceptible' persons, such as those investigated by Baron Karl von Reichenbach in his studies of somnabulism.

The Baron, enriched by his industrial chemistry discoveries, became convinced that somnambulism (sleep-walking) was due to the effects of moonlight and spent his time doing tests on 'susceptible' persons who told him they could see bright colours as crystals were hung between magnets in dark rooms.

As a child I was a somnambulist myself. My parents once discovered me in the middle of the night, searching through a draw, muttering "where's the sellotape?". Whether there was a full moon at the time, I don't know.

Another extraordinary thing about selenite is the size to which its crystals can grow. In 2000, there was a cover photograph on the Geological Society of London's Geoscientist magazine of people sitting in a cave surrounded by huge crystals. When I saw it, I must admit I assumed it was some kind of hoax. It would be so easy to superimpose images of geologists inside a geode using photoshop, wouldn't it?

However, this article from the BBC seems to confirm that such crystals do exist in a small cave near Almeria in Spain.

Also in 2000, Mexican miners discovered a cavern 300m below the ground which contained selenite crystals the size of tree trunks. The photographs are reminiscent of Superman's Kryptonite grotto.