Selenite is the crystalline form of gypsum and one of the softest decorative minerals sold, at about 2 on the Mohs scale — you can mark it with a fingernail. It is also slightly soluble, so it should never be washed: wipe it with a dry cloth and keep it away from damp.
The towers are cut as hexagonal prisms and polished on the sides, with the top sawn flat or left as a natural cleavage face. Because the material cleaves along planes, small flakes can come away at the edges. That is the nature of the mineral, not a fault in the piece.