Selenite

A Mohs hardness of 2 places selenite at the extreme soft end of the crystal spectrum — softer than a copper coin, softer than a fingernail, and in the same hardness range as common table salt. This physical fragility is worth dwelling on, because it tells you something essential about how to work with the stone. Selenite belongs to the gypsum group, a mineral family defined by the presence of water molecules chemically bonded within the crystal structure itself (the ·2H₂O in the formula denotes this). That structural water is part of why the mineral is so soft, and it is precisely why selenite must never be submerged — the water in the stone is in equilibrium with the dry mineral lattice, and liquid water upsets that balance by beginning to dissolve the surface. It is not a slow process you can interrupt; even a few minutes of soaking will etch the silky lustre irreversibly.

What selenite lacks in hardness it more than compensates for in optical character. The variety called satin spar — a fibrous, columnar form of gypsum — displays a chatoyant shimmer when cut en cabochon, a bright internal silkiness caused by light bouncing between the parallel fibrous crystals within the mass. The dramatic giant crystals of Naica, Mexico, are a different face of the same mineral: selenite blades up to twelve metres long, formed over hundreds of thousands of years in a flooded underground cave at a remarkably stable 58°C, conditions that allowed crystal growth so slow and so sustained that the result barely seems real. They are among the largest natural crystals ever discovered, and they are composed of precisely the same compound as the soft wand sitting on your shelf.