Crystallization is a Quirk that allows Hojo to grow a large number of crystals from his body, completely covering his skin with them.
The crystals are shown to be both durable and able to grow virtually instantly. There does not appear to be any limitations as to where on the body the crystals can grow from.
Despite their hardness, the crystals are not unbreakable. Hojo also keeps his head shaved so that the crystals can grow smoothly, implying that hair may hinder the crystallization process.