Rainbow Obsidian

Rainbow obsidian is a variety of obsidian, a natural volcanic glass, whose iridescent sheen comes from aligned layers of microscopic mineral inclusions within the glass. Rainbow obsidian displays bands of iridescent color - purple, green, gold, and sometimes blue - visible when polished and viewed at the right angle against a dark background. The effect is caused by thin layers of nanocrystalline inclusions (microscopic crystals of hedenbergite, a pyroxene mineral) aligned within the glass during flow.

These nanoscale layers act as a natural diffraction grating, splitting white light into spectral colors through thin-film interference - the same physics that creates color in soap bubbles, oil films, and the wings of morpho butterflies. The specific colors visible depend on the spacing between the layers, which varies across the specimen.

The finest rainbow obsidian comes from Mexico, where specific volcanic eruptions produced lava with the exact chemistry and cooling rate needed to create the oriented nanocrystalline layers. Not all obsidian from these flows shows the rainbow effect - it requires a precise balance of glass chemistry, cooling rate, and flow dynamics.

Rainbow obsidian is identified by its iridescent rainbow sheen visible on polished surfaces when viewed at certain angles against a dark background. The base material is black obsidian with standard glassy fracture and hardness 5.5.

Distinguish from labradorite (feldspar, different flash mechanism, crystalline), fire agate (botryoidal form, iron oxide layers), and regular obsidian (no iridescence). The rainbow sheen should show spectral colors that shift as the viewing angle changes.