These are BGO-12 aka Bismuth Germanate facet rough.
This is a real cool rarity in the gem world. Bismuth Germanate (in this case, specifically Bi3Ge4O12 aka evlitine) is valued as an optical substrate because of its highly unusual electrooptical properties. It has exceptionally good optical properties too, with (as far as we can tell the literature is weak) an RI of 2.15 and dispersion of 0.1. Cut stones are incredibly beautiful with high brilliance and incredible play of color. Its only real downside is its hardness of 5 and brittleness, requiring extra care in cutting. Note it also has a high density of 7.1. It's overall a super cool synthetic very rarely seen in the gem world.
