A cave pearl is a small, usually spherical,

speleothem A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending ...

(cave formation) found in

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

caves A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...

. Cave pearls are formed by a

concretion A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...

of calcium salts that form concentric layers around a nucleus. Exposure to moving water polishes the surface of cave pearls, making them glossy; if exposed to the air, cave pearls can degrade and appear rough.

Composition

A cave pearl is composed primarily of calcite (calcium carbonate aCO₃. Cave pearls are generally not considered to be a type of oolite. Other minerals found in small quantities in cave pearls include

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

(silicon dioxide iO₂, apatite (a group of

phosphate minerals Phosphate minerals contain the tetrahedrally coordinated phosphate (PO43−) anion along sometimes with arsenate (AsO43−) and vanadate (VO43−) substitutions, and chloride (Cl−), fluoride (F−), and hydroxide (OH−) anions that also fi ...

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

, and

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

Formation

Cave pearls form when water dripping into a cave loses

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

and precipitates calcite. A cave pearl forms when the water is moving too vigorously to form a stalagmite. A nucleus of matter (such as a grain of

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...

) becomes coated with calcite, and the

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

then provides a rotation to the nucleus in such a way that it is coated evenly. In this manner, concentric layers build up over time, in much the same way that a biological

pearl A pearl is a hard, glistening object produced within the soft tissue (specifically the mantle) of a living shelled mollusk or another animal, such as fossil conulariids. Just like the shell of a mollusk, a pearl is composed of calcium carb ...

forms within a mollusc. There may be microbial action involved in the formation of cave pearls. The existence of an actual pool may not be necessary for cave pearls to form, as long as the deposit is kept wet and agitated by water dripping or trickling through. If the cave pearl sinks to the bottom of a pool or is otherwise in direct contact with moving water, the motion of the water buffs it to a high gloss. Although the motion of the water often keeps cave pearls from adhering, sometimes cave pearls will stick to one another or to the bottom of a pool.

Nucleus

A cave pearl forms around a nucleus of matter. The nucleus of a cave pearl is typically very small, such as a grain of sand, but can be larger. Some nuclei are made of foreign matter (such as quartz sand,

wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...

, or even

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

), whereas others are made of calcified

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

or limestone.

Shape

Cave pearls are usually

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

but can also have other shapes. The reason cave pearls tend to be round is not due to erosion by rotation, but rather that their growth is steady and uniform. Because a spherical shape allows the greatest amount of deposition for the smallest surface area, a cave pearl tends to end up spherical even if its nucleus is highly irregular. Sometimes several cave pearls stick together to form a shape that resembles a bunch of

grapes A grape is a fruit, botanically a berry, of the deciduous woody vines of the flowering plant genus ''Vitis''. Grapes are a non- climacteric type of fruit, generally occurring in clusters. The cultivation of grapes began perhaps 8,000 years ago, ...

. In addition to the typical spherical shape, cave pearls can be cylindrical, elliptical, cubical,

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A '' regular hexagon'' has ...

discoid Discoid may refer to: * Disk (mathematics), the region in a plane enclosed by a circle Medicine * Furosemide, a medication sold under the trade name Discoid * Discoid meniscus, a human anatomical variant * Discoid lupus erythematosus, a chroni ...

, or irregular.

Size

Most cave pearls are smaller than wide. Large cave pearls grow as big as in diameter. The world's largest cave, Son Doong Cave in

Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making i ...

, has cave pearls "the size of baseballs".

Frequency

Cave pearls are relatively common in caves but are typically present in low abundance. In Tabasco, Mexico, the Gruta de las Canicas (Cave of the Marbles) is highly unusual in that it contains a tremendous quantity of pearls: an estimated 200 million pearls were discovered on the cave floor, in some areas to a depth of a meter or more. The mechanism for the formation of this vast quantity of pearls has not been determined. The Rookery, in

Carlsbad Caverns Carlsbad Caverns National Park is an American national park in the Guadalupe Mountains of southeastern New Mexico. The primary attraction of the park is the show cave Carlsbad Cavern. Visitors to the cave can hike in on their own via the natur ...

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ke ...

, has so many cave pearls that they were at one time handed out to visitors as souvenirs.

References

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External links

Conference Abstract: Microbes and Cave Pearls
Speleothems Limestone caves