Rainbow Moonstone with Black Tourmaline

The photo shows two white teardrop-shaped stones. Move them in the light, and a bluish glow appears. Inside there are thin black streaks, and one of them has a large black patch at its tip. Shops call this stone "rainbow moonstone with black tourmaline". Lovely, but the two key words in the name are not quite accurate.

Let's start with "moonstone". Classic moonstone is orthoclase, a potassium feldspar. According to Wikipedia, what gem lovers call "rainbow moonstone" is usually labradorite, a feldspar from a different group, the plagioclases. According to Geology.com too, its colors usually come from labradorite: white light splits into rainbow colors inside the stone. So it is not exactly moonstone, but it is definitely family, and it has the same glow.

Now for "black tourmaline". Black tourmaline is called schorl, and it is the most common kind of tourmaline. According to Wikipedia, 95% or more of all tourmaline in nature is schorl. But according to an analysis published by the Dutch site Stapel van Stenen, the black mineral found in rainbow moonstone from India was identified as biotite, a mica, and not tourmaline. We have not tested the stones in the photo in a lab, so we do not know what is in them. One thing is certain: there is something black in there.

So: one stone, and two words in its name that might need fixing. That takes nothing away from its beauty. A lot of rainbow moonstone comes from India, and according to a 2024 GIA article, fine rainbow moonstone has also been found in Madagascar. Keep in mind that labradorite, like all plagioclases, tends to split: according to Wikipedia, it has three directions of cleavage. So better not to drop it on a stone floor.

For the stories about the Romans, who thought moonstone was made of solidified moonbeams, and about Florida, see the grey moonstone page.