(Strand pictured isn't exactly what I bought, but the colors and quality matches well.)
Despite its trade name, Impression Jasper (also called Snakeskin Jasper or Aqua Terra Jasper) isn't technically a jasper. Suppliers have their marketing reasons behind naming things as they do, I guess; admittedly all of those names sound more romantic and poetic than Variscite (which is the "real," non-trade name appellation of Impression Jasper).
Anyway, Variscite is a hydrate—a new mineral formed by the interaction between water and an existing crystal structure. Like chalcedony, there's Variscite the mineral and variscite the mineral group. Variscite the mineral is AlPO4*(2H2O); the variscite group is characterized by the formula "A(X04)*(2H2O)," where A is either aluminum or a positive iron ion, and X is either phosphorous or arsenic.
Variscite in particular happens when water loaded with phosphate (PO4) runs into aluminum. It's most often found in deposits in Germany; in fact, the name "Variscite" is a reference to an older name for Vogtland, Germany, where it was first discovered in the 1830s.
Lots of suppliers are selling items listed as "Hot Pink Impression Jasper" and the like. This isn't a naturally occurring color in Variscite or in nature; the stone has been dyed or is otherwise unnatural. In nature, Variscite comes in beautiful shades of teal and green, and is often polished with bits of the brown or tan matrix mixed right in. Sometimes it gets mixed up with turquoise, or occasionally sold as "variquoise," in colors like you see below.
'Impression Jasper' by Kokoba
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