Etchings on a topaz crystal
Or not
The Thomas Mountains of Utah, USA, are famous for their topaz, and in fact topaz is Utah’s state gem. The rocks are volcanic rhyolite, the fine-grained equivalent of granite, erupted relatively recently, about 6 million years ago or so, but some may be as old as 25 million years, toward the end of the mid-Tertiary ignimbrite flare-up.
The magma must have contained an unusual amount of fluorine, because topaz’s chemical formula is Al2(SiO4)(F,OH)2. Without the fluorine on the end, some other aluminosilicate mineral would probably have formed rather than topaz.
This specimen has some flat hematite crystals in the lower part of the photo at top (the black things) but the most interesting aspect to me is the rectangular etch pits on the surface of the crystal. Because the pits are aligned with the edges of the crystals and reflect the orthorhombic symmetry of topaz, I think they are the result of some irregularity in the topaz crystal growth, rather than indicating some other mineral that grew on the surface and was later dissolved away. The pits are on a prism face of the topaz crystal.
Thus, even though they are usually called “etch pits,” I think that’s likely a misnomer, because it is really a matter of variation in crystal growth rather than etching.
A valid explanation rather than just etching assumptions.
For the life of me I can only ever see bumps, not pits, on that surface!