Volcanic gypsum

While sulphur in the solid state is immediately recognisable by its typical yellow colour, you can often see white crystals around the fumaroles.
These are gypsum crystals, also produced by the sublimation of the volcanic gases as they leave the surface. In fact, gypsum is formed half by sulphur and half by calcium held together by water.
When it comes out, the sulphur of the fumaroles can then join the calcium found in the surrounding soil, slowly forming the gypsum crystals