Sard is a translucent to semi-transparent variety of chalcedony. The colors are similar to carnelian but they are darker and less saturated in red brown to brown or dark orange.
Gem Reference Guide, Los Angeles: Gemological Institute of America, 1988. Pp. 45-48.
Newman, Harold. An Illustrated Dictionary of Jewelry, New York: Thames and Hudson, 1981. Pp. 269.
Schumann, Walter. Gemstones of the World, New York: Sterling Publishing, Co., 1977. Pp. 126.