Ulexite is an evaporite mineral that forms in alkali salt flats where the local water is rich in boron. It has a hardness of about 2 on the Mohs scale. In rock shops, cut slabs of ulexite like this one are commonly sold as "TV rock." It consists of thin crystals that act like optical fibers, so if you lay it on a paper, the printing appears projected on the upper surface. But if you look at the sides, the rock is not transparent at all.
This piece of ulexite comes from the Mojave Desert of California, where it is mined for many industrial uses. On the surface, ulexite takes the shape of soft-looking masses and is often called "cotton ball." It also occurs beneath the surface in veins similar to chrysotile, in which the crystal fibers run across the thickness of the vein. That's what this specimen is. Ulexite is named for its German discoverer, Georg Ludwig Ulex.
