One green and three black candles were brought to the Citadel from Valyria a thousand years before the Doom of Valyria. They are tall and twisted with sharp edges. These glass candles give off an unpleasantly bright light that does strange things to colors (white becomes as bright as fresh fallen snow, yellow shines like gold, reds turns to flame, and shadows become so black that they look like holes in the world).
Info Link: https://awoiaf.westeros.org/index.php/Glass_candle
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