One of the most ancient of the August birthstones, sardonyx is a mix of two kinds of chalcedony (cryptocrystalline quartz): sard and onyx. Bands of brownish purple to brown to darkish orange sard alternate with ordinarily white or black levels of onyx. Around the zodiac birthstones graphic charts (accessible by https://www.gemsbiz.com/blog/explore/birthstones-by-zodiac-sign