Iris stones
The rainbow inside the stone.
Iris agate and iris quartz are real gem names for stones that show rainbow colors. Here's what causes the rainbow, and why it's tricky to wear.
Iris agate and iris quartz
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Iris agate is a real term
Iris agate is an established mineralogical term. It means agate that shows rainbow (spectral) colors when a thin slice is viewed with light passing through it.
Source: American Mineralogist (Mineralogical Society of America), Vol. 37, “Iris Agate” by Frances T. Jones
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What makes the rainbow
The colors come from a natural diffraction grating: the edges of very thin layers inside the agate split light into a spectrum.
Source: American Mineralogist (Mineralogical Society of America), Vol. 37, “Iris Agate” abstract
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Where the name comes from
The name iris agate is used because one meaning of the word iris is a rainbow-like display of colors.
Source: Geology.com, Iris Agate
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Iris agate is tricky in jewelry
Iris agate's rainbow only shows when light comes from behind the stone, so it isn't a good material for many jewelry uses.
Source: Geology.com, Iris Agate
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Iris quartz is a recognized name
Iris quartz is a real name too. GIA's journal says naturally iridescent quartz has been known for over a century under names including iris quartz and rainbow quartz.
Source: GIA Gems & Gemology, Causes of Iridescence in Natural Quartz