Which magical glass candles from the novels were left out of Game of Thrones?

The magical candles from the novels that were left out of Game of Thrones are glass candles.

Glass candles are tall, sharp-edged candles made from obsidian, also called dragonglass. In the novels, the Citadel keeps four of them—three black and one green. Acolytes must spend a night attempting to light one before taking their maester's vows, a ritual meant to teach humility and remind them that some mysteries remain beyond human knowledge.

The candles become more than a ceremonial object. Archmaester Marwyn tells Samwell Tarly that Valyrian sorcerers used them to see across enormous distances, send visions and dreams, and communicate across the world. By the time Sam reaches Oldtown, a black glass candle is burning in Marwyn's study, suggesting that ancient magic is returning with the dragons.

Glass candles are often confused with ordinary dragonglass weapons, such as the obsidian daggers used against the Others. Both are made from obsidian, but glass candles are magical instruments, not blades. The television adaptation omitted this Citadel mystery from its main story.