The most historic with the August birthstones, sardonyx is a combination of two varieties of chalcedony (cryptocrystalline quartz): sard and onyx. Bands of brownish purple to brown to dim orange sard alternate with usually white or black levels of onyx. The birthstones can be found in the next chart, listing https://www.gemsbiz.com/blog/explore/birthstones-by-zodiac-sign