how to facet
gemstone main cabochon creation
min data

Mineral: quartz
Gem: fire agate (cryptocrystalline)
Cut: cabochon
Origin: Arizona
General Info: Agate (achates) was named from the river Achates, now the Drillo, in Sicily, where the stone was first identified. It is cryptocrystalline quartz (micro crystals) usually with a banded pattern (although not always).
Additional Information
Fire agate was created by the alternating deposition of hematite layers on chalcedony. The hematite (specular hematite) create the color from a surface oxidation caused by diffraction of light. The chalcedony (crytocrytalline quartz) layers over the hematite trapping the oxidized structure and protecting it. Stone cut by the author.
In these closeups taken with a microscope you can see the hematite layering and get an idea of the colors they produce. The hematite does not actually change to the various colors, but the surface oxidation is so uniform it creates diffraction patterns that produce the color display.

Hence the color seems to move around over the hematite surface as the stone is rocked. In this case (unlike opal) the light is reflected directly from the opaque surface rather than passing through it.