druse

Druse, also spelled drusy, is a geological term for a coating or lining of tiny crystals that densely covers the interior of a rock cavity or fracture, or a surface that is covered with such microcrystals. The crystals in a druse are usually well-formed but small, giving a glittering, sparkly appearance when light reflects from many facets. Common druse minerals include quartz and calcite, but druse can form with aragonite, dolomite, gypsum, celestite, and other minerals depending on the chemistry of the fluid from which they crystallize.

Druse forms when mineral-rich fluids infiltrate voids in rocks such as geodes, vugs, nodules, or cavities created by weathering. As the solution cools or its chemistry changes, minerals precipitate and crystallize on the cavity walls, often over long periods. The result is a crust or veil of microcrystals that may cover all interior surfaces or only portions of them.

Geodes in volcanic or sedimentary rocks frequently exhibit druse quartz or calcite lining; amygdaloidal basalts and limestone vugs are common settings for druse textures. The term can also describe draped layers of microcrystals on the exterior of a polished rock when crystals form and cling to surface facets.

In mineral collecting and lapidary work, druse is a prized texture because of the sparkle produced by countless tiny faces. When the crystals are exposed on a surface rather than interior cavities, the texture is described as drusy.