Sard 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.
Sard is the uniformly colored red-brown or brownish-yellow variety of Chalcedony, which in turn is a microcrystalline variety of Quartz.
There is no strict separation between sard and cornelian/carnelian (red to reddish brown), because they are both chalcedonies found in the same deposits and were employed for the same objects.
The tints of a sard stone may be reddish or orange. Good-quality material is blood-red when light is passed through it, and blackish red in reflected light. The distinction between sard and cornelian is a matter of individual judgment, and one is taken for the other, but usually sard is a bit darker than cornelian. The colour of sard and cornelian is caused by the dispersion of very tiny particles of hematite, one kind of iron oxide. And if stones contain disseminated iron compounds, heating will oxidize them.
Other reddish chalcedonies may have been coloured this way.
All chalcedonies form in cavities of rocks of different types, especially lavas, and develop at relatively low temperatures, as a solid substance formed from silica-rich solutions. It can also be formed from the dehydration of opal, a hydrous stone which loses water as it ages.
According to the writings of the Roman naturalist Pliny the Elder, sard is named after a town in Asia Minor, Sardis in Lydia. But another opinion is that the name comes from the Persian "sered" meaning "yellowish-red," which could be more likely as sard was used many centuries before the Roman period.
It is the first foundation stone (sardius) indicated in the breastplate of the Hebrew high priest, also known as Aaron's Breastplate. It was also used to make Egyptian scarabs, used as commemorative badges for historical events, or as beetle-shaped amulets that represented their belief that like the scarab dung beetle, the sun rolled in similar fashion making changes in human souls. In Assyria, sard was one of several hard stones used for cylinder seals, to make cuneiform marks on wet clay tablets or print images on cloth, illustrate events, or affix signatures to official documents, in the same way that we use rubber stamps today.
In the Greek and Etruscan civilizations, sard was used to make engraved gems, luxury items which were not only polished gemstones, but intricate art designs or profile portraits carved right into the stone using a hand-drill and powder from harder stones.
Since about 400 B.C. the best sard and cornelian has come from India. But today it comes from many other sources. Most of the material sold commercially comes from different deposits in Brazil, USA, Sri Lanka, and Uruguay.
All varieties of chalcedony are cut into cabochons, engraved, or made into seal stones or rounded, polished, and pierced for necklaces and other items of jewelry. Sard is an ideal seal stone, because the hot wax used to seal ancient letters doesn't stay stuck on the stone or signet ring.
Artificially-colored sard is produced from chalcedony by saturating it with sugar solution.
Over the years, simple heat treatment regimes have been devised to produce such commercially acceptable permanent colour changes in gems and gem materials as converting carnelian to sard.