A glass candle is a candle made of obsidian. Historically they burned, but they have not been reported to burn in recent memory.
Voluspa®, Candles, Diffusers & More
Wyndham Catacombs
Birthday Cake Cookie's Dazzling Candle Party - Cookiepedia
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Fine Fragrances & Scented Candles
ENSTAKA scented candle in glass, Bonfire/gray, 40 hr - IKEA CA
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Ice hotel - Wikipedia
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ENSTAKA scented candle in glass, Bonfire/gray, 50 hr - IKEA CA