File:Geyserite eggs (Sawmill Geyser, Upper Geyser Basin, Yellowstone, Wyoming, USA) 6 (21639420239).jpg

Geyserite from the Holocene of Yellowstone, Wyoming, USA.

Geyserite (a.k.a. siliceous sinter) is a rare, friable to solid, chemical sedimentary rock composed of opal (hydrous silica, a.k.a. opaline silica: SiO2•nH2O). Geyserite forms by precipitation of hydrous silica from hot spring or geyser water. At the Yellowstone Hotspot Volcano in Wyoming, the source of the silica is subsurface rhyolite lava flows of late Cenozoic age. Superheated groundwater moves along fractures in the rhyolite and leaches out silica. When hot spring or geyser water reaches the surface, the water cools and evaporates, resulting in precipitation of very thin layers of opal on any substrate. Numerous precipitation episodes (for example, from repeated geyser eruptions) results in the buildup of geyserite.

The geyserite specimens shown above are called a "geyser eggs". Geyser eggs are unattached masses of geyserite that occur in some geyser splash pools and proximal runoff pools. Good examples of Yellowstone geyser eggs (a.k.a. geyser beads & geyser pearls) are present at Aurum Geyser, Sawmill Geyser, Bead Geyser, and Narcissus Geyser. Early tourists to Yellowstone often collected these objects - a type of vandalism. Such vintage specimens very rarely appear in the modern retail market.