A shimmering or iridescent play of light on certain minerals and crystals, creating a sparkly or pearly effect.
From German 'schillern' meaning 'to shimmer.' It's a technical term borrowed directly from German mineralogy in the 1800s to describe optical effects in stones.
Schiller makes some minerals look like they have inner light—it's why moonstone looks magical and labradorite shows those mysterious flashes of color. The effect happens because light bounces through layers inside the crystal, creating the same magic you see in oil slicks or butterfly wings.
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