Diana Craig Patch
To my mind, carnelian is one of the prettiest of the semi-precious stones. The ancient Egyptians thought so too and used the red stone to make jewelry beginning in the Predynastic Period, as early as 3800 B.C.
Carnelian is a variety of chalcedony, with iron oxide inclusions that create its striking orange, red, or red-brown colors. Chalcedony is a form of quartz that has an almost invisible crystalline structure –known as cryptocrystalline or microcrystalline− unlike other well-known types of quartz, such as amethyst or milky quartz, which have macro crystals. This cryptocrystalline structure is shared by other types of quartz besides chalcedony, including agate (beads), flint or chert (tools), and jasper (rings and sculpture). The Met’s famous fragment of a face of a queen, possibly Nefertiti, is fashioned from jasper (http://www.metmuseum.org/collection/the-collection-online/search/544514?rpp=30&pg=1&ft=Egypt+queen&pos=16). The cryptocrystalline structure allows for all of these stones to be successfully knapped into objects because the stone fractures in predictable ways. Macrocrystalline quartz is significantly more difficult to work.
Carnelian was formed as solutions of silica that solidified into clumps in shallow ancient oceans in developing limestone beds. Chert and flint were often deposited in the same way. The source of ancient Egypt’s carnelian is largely unknown although one mine has been identified, Stela Ridge, located at Gebel el-Asr, west of Lake Nasser. According to James Harrell (http://www.escholarship.org/uc/item/57f2d2sk#page-2), this source was not active in the New Kingdom. Mostly it is thought that the carnelian was found as large, dark brown cobbles in Nile terrace gravels from southern Egypt and in Sudan where the cobbles have eroded out of limestone or sandstone deposits.
When a carnelian cobble is hit hard with a rock or hammer, it breaks revealing an interior with shades of red that is sometimes banded. Carnelian can be quite translucent if a thin enough piece is struck from the core (the cobble). Sard is a similar stone, but tends to be more opaque and brown in color, while jasper is an opaque dark red or yellow. The ancient Egyptians called carnelian herset. It is not clear what this word meant to them, but they considered red a powerful color with strong associations to blood and the desert. That this semi-precious stone was important to the ancient Egyptians may be seen in its presence on lists of materials and tribute in inscriptions, including an important list from the temple of Dendera.