Carnelian is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of silicon dioxide, their full chemical compound being SiO2.
Carnelian is a variety of chalcedony, which is translucent and is red to reddish-brown. It may also occur as flesh-colored chalcedony, known as cornelian or carnelian. It is slightly similar in appearance to its relation, the dark brown chalcedony, sard.
The colour of cornelian is caused by colloidally dispersed hematite. Other reddish chalcedonies may have been colored by heating since if they contain disseminated iron compounds the heating will oxidize them. Over the years, simple trade-accepted heat treatments have been invented to convert carnelian to sard, transforming dull gemstone colors into more marketable hues.
Chalcedony is a compact form of silica, composed of microscopic quartz crystals. It is softer than quartz and denser than opal. Its appearance may range from transparent to translucent to opaque.
It occurs in some fossils, such as petrified wood. It forms in cavities in rocks of different types, especially lavas, and develops at relatively low temperatures.
Visible examples would be the carnelian in Uruguay and California. Most are agates colored with ferrous nitrate solution. When held against the light, the colored variety shows stripes, while the natural variety has a cloudy distribution of color.
The best qualities of carnelian come from India (also called "cambay stone"), where the brown tints are enhanced to red by exposure to the sun. Yellow carnelian is called "canary stone."
Carnelian (or cornelian) derives its name from the Kornel type of cherry because of its color. In antiquity it was thought to still the blood and soften anger.
India has produced most of the finest sard and cornelian since the fourth century BC but there are many other sources worldwide. Much commercial material comes from various deposits in Brazil and Uruguay. Some beaches on the east coast of England produce cornelian.