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The gemstone crystals found in the lunar surface dust are tiny grains of sand, but they definitely are there. Any large crystals that ever existed on the surface would have been puliverized by four billion years of meteor bombardment. However, let's look just below the surface.
Consider these two chemical recipes for rather interesting minerals:
Corundum Spinel
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2 parts aluminum 1 part magnesium
3 parts oxygen 2 parts aluminum
4 parts oxygen
If you mix these ingredients at the right balance of temperature, pressure, and gravity, you get regular, orderly crystals. In general, the lower the rate of cooling or the lower the gravity, the larger the crystals will grow. So we have a big unknown. Which was more influential in crystal growth on the moon, as compared to terrestrial minerals? Its rapid cooling, or its lower gravity?
We don't know.
But we do know that if you take the crystalline lattice of corundum and sprinkle in just a few atoms of chromium, it will glow bright red; and you have a ruby.
If you try too hard and put in too much chromium, you'll get a different crystalline lattice when the melt cools. You'll spoil the pretty red color, and get green instead. But take heart, it's an emerald.
If instead you toss in a few atoms of iron or titanium, you'll get a sapphire. The color of the sapphire depends on how much of each of these spices you put into the soup.
My guess, based on what little chance I've had to study geology and methods for producing synthetic gems, is that the lower gravity of the moon is the overwhelmingly important factor in the formation of crystalline gemstones. Humans have made synthetic rubies, emeralds, and opals in their laboratories by mixing up the right ingredients and allowing the melt to cool for a period of hours. While it's true that the moon apparently cooled much faster than the earth, we are still talking about a period of millions of years.
After its initial cooling the moon was subjected to some rather dramatic remelting events such as the impact that created the great Imbrium basin. There have been repeated opportunities for the formation of crystalline minerals in the moon. Whether anything fortuitous occured during any of those opportunities is a question that remains for the future.
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