Usually when you buy a scented candle the scent is released from the wax as it burns, however there are others available which have a twin layer of glass, the internal “cup” containing the candle and the outer “cup” containing a gel which, when warmed by the internal cup candle, releases the scent.
Over Christmas my parents had purchased one of these and had it burning on the coffee table, releasing seasonally inspired scent into the living room. All of a sudden a large, tall flame was observed. While it was possible to extinguish the flame simply by firmly blowing on it, the flame was quite large and could have ignited other objects surround the the candle on the table. When extinguished it was clear what had happened; the outer layer of gel had somehow ignited and had started burning at a furious rate (shown in the photo below)
Having mentioned this to friends I’m now aware of at least one other of these types of candles which has caught fire, as such I’d recommend anyone that has a candle of this type not to use it and to dispose of it for safety.