Crystallization is a Quirk that allows Yu 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. It does not appear to have any limitations to where on the body that the crystals can grow from. Yu Hojo also possesses the ability to form his crystals into a sword shape that covers his arm in order to extend his reach.
Despite their hardness, the crystals are not unbreakable. Yu also keeps his head shaved so that the crystals can grow smoothly, implying that hair may hinder the crystallization process. The crystals also appear to burst through whichever clothes he is wearing.