Cave pearl
Encyclopedia
Speleothem
A speleothem , commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems are typically formed in limestone or dolostone solutional caves.-Origin and composition:...
, a concretion of calcium salts which forms in limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
caves. They may be any shape but are formed of concentric layers, reaching up to 15 cm (5.9 in) in diameter. Most are smaller than 1 cm (0.393700787401575 in) wide. They can be found singly or grouped. A cave pearl forms as calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
crystallizes on a nucleus such as a grain of sand, in much the same way that a biological pearl
Pearl
A pearl is a hard object produced within the soft tissue of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other...
forms within a mollusc
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...
. A cave pearl is formed when a current of water provides a rotation to the nucleus in such a way that it is coated evenly. In this manner it may form a sphere or a cylinder or any of several other shapes. The cave pearl then sinks and is buffed to a high gloss by the motion of the water. It sinks to the bottom of the pool, and while the motion of the water often keeps it from adhering, several cave pearls may stick together to form stone which looks like a bunch of grapes. Cave pearls often degrade when allowed to dry. There may be microbial action involved in the formation of cave pearls. Cave pearls are generally not considered to be a type of oolite
Oolite
Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites...
.