Carnelian, a form of chalcedony, is either opaque or translucent and always glassy.
It comes in a fabulous range of colours - yellow, gold, orange, paprika and brick red. Carnelian gets its colour from iron.
It is easily heat teated - even from the sun's heat - and will deepen in colour when exposed to heat.
Carnelian - also spelt cornelian - is a semi-precious gemstone often mixed up with sard. Sard is darker and harder but the two gemstones have become interchangeable.
Carnelian has been used in jewellery since the Romans used it for signet rings and seal rings for use in sealing wax on important documents.