The most historic of the August birthstones, sardonyx is a combination of two varieties of chalcedony (cryptocrystalline quartz): sard and onyx. Bands of brownish pink to brown to dim orange sard alternate with generally white or black layers of onyx. Having said that, the blue colour originates from a treatment https://www.gemsbiz.com/blog/explore/birthstones-by-zodiac-sign