Ruby on zoisite rock. Ruby is a red-coloured variety of the mineral corundum, a crystalline aluminium oxide mineral. It is very hard, so used as an abrasive in industry, and clear samples can be cut into ornamental stones, as here. This sample is a round, facetted ruby measuring 12 millimetres across. It is resting on a rock of zoisite mineral, a silicate of calcium and aluminium. The red areas in the rock are rough ruby.