Gemstone (in)Formation: Mookaite

Gemstone (in)Formation: Mookaite

BY : Rhiannon Nevinczenko

Mookaite (pictured) has a fascinating formation story. It is named for Mooka Creek, where it is mined (geologically known as the Windalia Radiolarite formation). Radiolarite is a type of sedimentary rock whose composition is primarily microscopic radiolarian remains. Radiolarians are single-celled organisms with skeletons made of silica. Mookaite is a type of radiolarite - a fossiliferous sedimentary rock.
During the early Cretaceous period, the Windalia Radiolarite formation (located in what is today western Australia) was under a shallow sea. Radiolarians with opaline silica skeletons were deposited as sediment, which accumulated. The sediment was slowly cemented into solid rock as minerals from groundwater seeped into the sedimentary layers, replacing the original organic materials. Thus, microscopic quartz crystals were deposited into the rock structure. The range of colors and patterns seen in mookaite is owed to mineral impurities and trace elements that snuck in during the mineralization process.
Mookaite is commonly called mook jasper, though it is not true jasper. This may be because both stones share a common feature: chalcedony (a cryptocrystalline form of quartz). However, while jasper is chalcedony with inclusions, mookaite (with its unique formation history) can be comprised of chalcedony, chert, opalite, or a combination of the three (though it is usually multi-colored chalcedony).
Mookaite is commonly favored due to the wide range of colors it can contain, and its high polishability. One string of mookaite beads might include everything from red, to white, to cream, to brown, to purple, to a marbled mix of two or more colors on just one bead!
Photograph credit: James St. John via Flickr. CC BY 2.0.

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