A bunch of neat optical effects in minerals/mineraloids/rocks

Ammolite, thin-film interference from the shell's nacreous structure (this is a mineraloid)

Another example of ammolite, this time showing its origin (an ammonite)

Pleochroism in andalusite

Pleochroism in axinite

Pleochroism in cordierite aka iolite

Pleochroism in epidote

Diffraction caused by the microscopically-banded structure of this agate results in vivid iridescence (agate is a type of rock)

Pleochroism in kornerupine

Interference of light caused by tiny exsolution lamellae of labradorite in this cabochon (technically a rock as the exsolution lamellae are labradorite within a labradorite-depleted matrix)

The same effect, so-called "labradoresence," in an unpolished slab

The opposite side of the same slab, polished

Pleochroism in tanzanite (top) and tourmaline (bottom)

Welo opal gets its iridescent fire from diffraction within its structure of packed silica spheres (opal is a mineraloid, not a mineral)

The sheet-like crystal growth in seraphinite (left) and charoite (right) cause a silky subsurface sheen that moves along the surface of the stones

Pleochroism in tanzanite