A cave pearl is a small, usually spherical, speleothem (cave formation) found in limestone caves. Cave pearls are formed by a concretion 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 Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

(calcium carbonate aCO₃. Cave pearls are generally not considered to be a type of

oolite Oolite or oölite (''egg stone'') is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Ancient Greek word for egg (ᾠόν). Strictly, oolites consist of ooids of diameter 0.25–2 ...

. Other minerals found in small quantities in cave pearls include quartz (silicon dioxide iO₂,

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

(a group of phosphate minerals), iron, aluminum, and magnesium.

Formation

Cave pearls form when water dripping into a cave loses carbon dioxide and precipitates calcite. A cave pearl forms when the water is moving too vigorously to form a

stalagmite A stalagmite (, ; from the Greek , from , "dropping, trickling") is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from ceiling drippings. Stalagmites are typically ...

. A nucleus of matter (such as a grain of sand) becomes coated with

, and the current 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 forms within a

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

. There may be

microbial A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

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,

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, an ...

, or even plastic), 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 part ...

or limestone.

Shape

Cave pearls are usually spherical 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 The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc ...

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

cylindrical A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infini ...

, elliptical, cubical, hexagonal, discoid, 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 A son is a male offspring; a boy or a man in relation to his parents. The female counterpart is a daughter. From a biological perspective, a son constitutes a first degree relative. Social issues In pre-industrial societies and some current co ...

in Vietnam, 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, New Mexico, 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