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Conch Pearl

May 14, 2020

Conch "pearls," calcareous concretions produced by the Queen conch mollusk, formerly known as Strombus Gigas or more recently as Lobatus Gigas, has mostly a very attractive pink color that may be enhanced by a characteristic silky-looking "flame structure". Some conch "pearls" show a smooth and shiny porcelain-like luster that makes very attractive.

 

Although they are truly rare gems, which are found only in the waters of the Caribbean and Bermuda and which seldom occur in quality and size suitable for jewelry. The odds of finding a conch “pearl” is 1 in 10,000 conch shells. Large unmounted conch "pearls" of high quality have sold for significant prices.

   

Because they are non-nacreous which means they are not made of nacre, the substance that gives traditional pearls their iridescent lustre, conch cannot be considered true pearl. Consequently, the term pearl, commonly used in the trade to describe this material, appears in quotation marks. The name Conch is pronounced "conk”.

 

BIOLOGY OF STROMBUS GIGAS MOLLUSK

 

Only Lobatus Gigas, a univalve mollusk commonly known as the Queen conch, grows the calcareous concretions known as conch "pearls." However, one must be careful because the term conch is sometimes used to describe other kinds of shell. Nevertheless, fishermen and divers can easily distinguish Lobatus Gigas from other snails by its distinctive “hook” or "claw.”

 

Reproduction takes place during the warmer months, from March to September. After mating, the female produces an egg mass of about 500,000 units, which is camouflaged from predators by a coating of sand. Four to five days later the eggs hatch and a tiny shell-less conch, called at this stage a "veliger," emerges to drift in water for about four or five weeks, subject to currents and extensive predation, until it settles to the bottom and acquires a small white shell. The young conch, now called a "creelzer," seeks protection in the sand. Over the next two years, it will grow its spiral shell, at which time it becomes a "roller." At about three years of age, the mollusk begins to build up its shell in a flared lip. At this point, it has reached breeding maturity and is at optimum size for fishing. It also seems that this is when the snails begin to produce "pearls." The "sangat' or "samba" is a fully matured conch, with a very thick, leathery-looking shell. A group of conch shells of various ages from roller to sanga.

  Photo by: Flower Garden Banks Marine Sanctuary  

GEMOLOGY OF CONCH “PEARL”

 

Size

 

Calcareous concretions from the conch shell are commonly more than 2-3 mm in diameter and around 0.2 or 0.3 ct in weight. Only rarely do they occur in sizes and of a quality suitable for use in jewelry. Although specimens up to 10ct are found, larger "pearls" are exceptional.

 

Shape

 

Conch "pearls" occur in a variety of shapes, ranging from extremely baroque to very symmetrical. They are generally somewhat rounded, but only very rarely are they spherical.

 

Fading

 

Unfortunately, the attractive color of pink conch "pearls" is not stable. Like the shell itself, these "pearls" fade on prolonged exposure to sunlight to a much lighter pink. This characteristic of the shell was noticed early on, as a consequence of its use for cameos.

 

The fading is probably related to the organic origin of the pink color. GIA obtained the absorption spectrum of a pink conch "pearl" on a Pye-Unicam UV-visible spectrophotometer. A broad absorption band centered around 500 nm is responsible for the pink coloration. Studies using Raman spectros- copy have demonstrated that the intensity of this visible absorption is related to the intensity of certain lines in the Raman spectrum. These lines are characteristic of organic compounds of the carotenoid family, to which a large number of organic pigments belong. The exact nature of the pigment(s) is not known. Inasmuch as many organic products fade, it is not surprising to observe this phenomenon in the conch "pearl." The fading is probably caused by the decomposition of the product when exposed to the ultraviolet rays of natural daylight. There is no known method by which the color can be restored. Consequently, fine pink conch "pearls" set in jewelry should be reserved for evening or occasional, as opposed to daily wear. With care, however, the color will remain strong, as the "pearls" in the Edwardian and Art Nouveau pieces.

  Edwardian piece (circa 1900) 23.5ct conch "pearl" set in a hinged platinum cage

Edwardian piece (circa 1900) 23.5ct conch "pearl" set in a hinged platinum cage

 

Flame Structure in Conch "Pearls"

 

One of the most striking characteristics of the conch "pearl" is the flame structure. In some cases, the surface reveals a regular pattern of parallel elongated crystals that imparts a silky sheen to the "pearl." In the very best specimens, the "flames" can be identified by microscopic examination as thin lamellae that are almost parallel to one another and are sometimes perpendicular to the axis of the "pearl," thereby giving rise to a rough chatoyant effect. Flame structure has also been observed in portions of the conch shell itself.

  From GIA  

Hardness and Toughness

 

We found the conch "pearls" to be of surprisingly high, yet variable, hardness for their composition. Scratch tests using hardness points from two different manufacturers were consistent and resulted in values between 4 and 5 for the two brown "pearls" and between 5 and 6 for two pink ones. This resistance to scratching explains why these "pearls" keep a very high luster even when worn in jewelry. We cannot currently explain why the hardness values are higher than those of both calcite (3)and aragonite (3.5-4).

 

Jewelers who have drilled and set conch "pearls" have been surprised by their toughness, which can be rated as good, making conch "pearls" relatively easy to set in jewelry.

  Conch Pearl Ring  

Conch "pearls" are truly rare and fascinating gems, which combine the still-unsolved mystery of their flame structure with their beautiful pink color and exotic origin. Apparently, conch “pearls” are very difficult to culture or to imitate which makes its value even higher. They are a true natural gem. Their hardness and toughness make them easy to mount and preserve in jewelry. After years of relative obscurity, they seem to have returned to fashion, probably because more are available, although they are still quite rare.

 

Source: GIA.edu