Carnelian, sometimes spelled cornelian, is a red or reddish-brown variant of chalcedony. Having the same chemical make up as Agate, when Carnelian grades into brown it is known as Sard. When it contains bands of white, it is known as Sardonyx.
In addition, some very dark brown Carnelian can be classified as Jasper. The distinction between these is very fine and relies more on visual appearance than scientific analysis. As a result, it is quite possible that one man's Carnelian may be another man's Sard, Agate or Jasper. Chemically, Carnelian is composed of Silicon Dioxide, SiO2, and gets its red colour from Haematite (Iron-Oxide, FeO2) impurities.
The colour of Carnelian is more intense in sunlight. Whilst some belief that the name comes from the Latin for flesh, it could also originate from 'cornum' which means cherry. And the slightly milky translucent red is very like the surface of this fruit.