 Philip King, Robert B. Neuman, Jarvis B. Hadley, chalcedony is sometimes considered separately from chert due to its fibrous structure.Since many cherts contain both microcrystalline and microfibrous quartz, it is sometimes difficult to classify a rock as completely chalcedony, thus its general inclusion as a variety of chert.The cryptocrystalline nature of chert, combined with its above average ability to resist weathering, recrystallization and metamorphism has made it an ideal rock for preservation of early life forms.