Wilkie Collins provides the reader with quite a detailed description of the titular gemstone, mainly through Betteridge’s reporting of the bequest left to Miss Rachel by her dissolute uncle, and it makes for very interesting reading. According to Betteridge, the Moonstone is:
“… It was a diamond! As large, or nearly, as a plover’s egg. The light that streamed from it was like the light of the harvest moon. When you looked down into the stone, you looked into a yellow drop that drew your eyes into it so that they saw nothing else… We set it in the sun, and then shut the light out of the room, and it shone awfully out of the depths of its own brightness, with a moony gleam, in the dark.”
There are a few things about this description which strike me as odd, but the colour of the stone isn’t one of them. Yellow diamonds are not unusual, their distinct pigment being caused by traces of nitrogen in the matrix (pure unadulterated carbon diamonds are colourless) and can vary between pale yellow to bright, vibrant canary yellow. A number of very famous diamonds have been yellow – below is the Oppenheimer, the largest uncut yellow diamond so far found, currently in the Smithsonian.
What bothers me about the description given of this gem by Betteridge is its apparent ability to phosphoresce. Glowing in the dark is not something one would normally associate with diamonds, although it is a phenomenon shared by some minerals, kunzite being an example. The brilliance of a diamond depends entirely on its dispersion of light, which is enhanced by the cut of the stone – see from the Oppenheimer diamond how dull it can appear when uncut and compare it with the cut De Allnatt diamond below:
But neither of these diamonds are known to phosphoresce and Collins’ assertion that the Moonstone does puzzles me. Is this a hint of the stone’s background as a sacred item, by giving it an otherworldly power not shared by other diamonds, or just a plain old mistake? Thinking about it, I have wondered if in fact this was a canary kunzite. Although usually a pink stone, yellow kunzites have been found and it is very likely that in the Victorian period it would have been easily mistaken for a diamond by anyone who wasn’t a trained gemmologist. This is what one looks like.
The fact also that the stone is looted would mean that it hasn’t been subjected to any geological or gemmological authentication, furthering the mistake. So as far as I’m concerned, the Moonstone isn’t a diamond at all. It’s a canary kunzite.
As an aside, it is also apparently a loose stone, but Collins gets round this by having Franklin fashion a setting from wire to allow Rachel to wear it as a brooch at her birthday dinner. But a stone the size of a plover’s egg would soon outweigh a basic wire setting and would, I expect, be much too large to wear as a brooch lest it tear the delicate fabrics of an evening dress. It would need proper setting and would most likely be best worn as a pendant. That said, there is evidence elsewhere in the book that the stone is not actually faceted – so why wear it at all?