Cairngorm is a variety of quartz crystal originally found in the Cairngorm Mountains of Scotland. It usually has a smokey yellow-brown colour, though some specimens are a grey-brown. Like other quartz gems, it is a silicon dioxide crystal, with a small amount of ferric oxide impurity which gives it the characteristic colour. It is used in Scottish jewellery and as a decoration on kilt pins and the handles of skean dhus. The largest known cairngorm crystal is a 52-pound (23.6-kilogram) specimen kept at Braemar Castle.