The Collaborative International Dictionary
Opal \O"pal\, n. [L. opalus: cf. Gr. ?, Skr. upala a rock, stone, precious stone: cf. F. opale.] (Min.) A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity.
Note: The precious opal presents a peculiar play of colors of delicate tints, and is highly esteemed as a gem. One kind, with a varied play of color in a reddish ground, is called the harlequin opal. The fire opal has colors like the red and yellow of flame. Common opal has a milky appearance. Menilite is a brown impure variety, occurring in concretions at Menilmontant, near Paris. Other varieties are cacholong, girasol, hyalite, and geyserite.
Wiktionary
n. (context mineral English) A type of stone, a mixture of quartz and opal deposited by a geyser as it precipitates out of the boiling water.
Wikipedia
Geyserite is a form of opaline silica that is often found around hot springs and geysers. Botryoidal geyserite is known as fiorite. It is sometimes referred to as sinter.
Usage examples of "geyserite".
Algae did the rest, painting the geyserite in all the hues of the rainbow.
The geyserite implied volcanism beneath, and also intermittent steam eruptions, which might have been what they had seen the night before.
Ojo Caliente was constantly being reshaped and rebuilt, in places spongy, in other places cracked and hard and brittle, the stuff of geyserite: a hydrous form of silica, a variety of opal deposited in gray and white concretelike masses, porous, filamentous, and scaly.