After I’d posted speculating that “Rainbow Garnet” was another name
for “Watermelon Garnet a.k.a. Transvaal Jade a.k.a.
hydrogrossular/idocrase,” I remembered what it probably really is.
But my computer was down and I couldn’t post.
The African hydrogrossular is always cabbed and is currently “new”
(again) on the market, and that’s what made me think of it. Then I
remembered another type of garnet, other than star garnet, that’s
always cabbed: an unusual type of grossular/andradite from Mexico and
one location in Nevada. The best of these stones throw off a bright
play of colors in reflected light that could give rise to the
trade-name “Rainbow Garnet.”
I believe the material currently being sold is coming from an
andradite location in Mexico. The only data I have is on the Nevada
material. It is honey-colored to dark brown and the reference I have
says the play of color is caused by lamellar twinning, a structure
somewhat similar to the twinning in microcline feldspar that causes
the “rainbow” in Labradorite.