Titanium Aura Quartz Cluster, Rainbow Coated Crystal Specimen, 1–2 in — Brazil

Rough quartz points from Brazil carrying a microscopically thin titanium layer bonded onto the crystal faces in a heated vacuum chamber. That coating is what produces the rainbow iridescence — the quartz underneath is ordinary clear quartz.

These are coated, not naturally coloured. The bases were left uncoated, so they stay clear quartz. Size and colour spread vary from cluster to cluster.