These Dakota Stones sardonyx 18x35mm faceted trapezoid beads are full of geometric style. Available in 8-inch strands, these beads feature stripes of white color against a black background, a natural geometric look that complements each bead's trapezoidal shape perfectly. Sardonyx is an onyx that is characterized by parallel lines of sard and different colored minerals. It is a distinct and dramatically banded variety of chalcedony composed of quartz layers. Sardonyx is believed to improve memory and increase one's analytical talents. Sardonyx is mined all over the globe and is also known as tuxedo agate. These beads would look extraordinary in a bold necklace design filled with geometric patterns.