Carnelian

Carnelian (also spelled cornelian) is a brownish-red mineral commonly used as a semi-precious gemstone. 
Similar to carnelian is a darker and harder stone called 'sard'. The two names are often used interchangeably. Both carnelian and sard are varieties of the silica mineral chalcedony coloured by impurities of iron oxide.
The gemstone has been used since prehistory.