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Geosciences LibreTexts

16.51.01: Sea of Cortez Pearl

  • Page ID
    4950
  •  

    Pearl
    Chemical composition Calcium carbonate, conchiolin and water
    Crystal system Amorphous
    Hardness 2.5-3.5
    Refractive index 1.52-1.69
    Specific gravity 2.68-2.86
    Lustre Pearly

    File:Esfericas3.jpg

    Figure \(\PageIndex{1}\): Sea of Cortez pearls®
    Photo Courtesy of Douglas McLaurin Moreno, Perlas del Mar de Cortez

     

    "Perlas del Mar de Cortez" or "Sea of Cortez Pearls" are also known under the commercial name of "Cortez Pearl", a trade-name owned by Columbia Gem House-Tri-Gem Designs. These pearls are produced in the Gulf of California's coastal city of Guaymas, Sonora, Mexico, inside a tranquil Bay known as "Bacochibampo Bay". The pearl farm is owned by "Cultivadores Mexicanos de Perlas S.C.L." since 2004, and was previously owned by the ITESM Education System. It originated as a humble school project (1993) led by four researchers: Enrique Arizmendi Castillo, José Manuel Nava Romo, Douglas McLaurin-Moreno and Sergio Farell Campos. Further funding from ITESM helped to expand this operation into a commercial venture in the year 2000.

    The farm is unique for many reasons, but mainly for being the only pearl farm in the world to utilize a pearl oyster of genus Pteria to produce its cultured pearls. All other marine pearl farms employ Pinctada genus species to grow their pearls.

     

    Color

    The pteria sterna mollusk is the species used to grow this unusual pearl. The common names for this species are: "Rainbow lipped pearl oyster", "Western Winged Pearl Oyster", "Concha Nácar" and "Callo de árbol". This oyster (Family Pteriidae) is capable of producing pearls in many colors -ranging from white to black, but gray, purple, lavender and blue are more common than white or black- and with varied overtones (such as blue, green, violet, purple and golden). The pearls are sometimes described as "opalescent" or as "abalone pearls" because of their unusual coloration. This coloration is due to several factors, one being its extremely good nacre coating, and the other one is a series of patterns that form on the surface of a pearl. When seen under a microscope these will resemble a human “finger-print”. Mexican Pearls display two features: an unusually compact structure of its aragonite crystals –seen at 500 nanometers- and the display of “spiraled” sub-patterns, which are unique to these pearls.

     

    Shapes

    Round and near-round shapes are exceptionally rare and account for less than 3% of a harvest. Semi-baroque (symmetrical: ovals, drops, buttons) shapes are more common with 25% and baroque (asymmetrical) shapes account for 71% of a harvest.

     

    Sizes

    The average size of the Sea of Cortez pearl is 8.9 mm, ranging in size from 8.3 to 9.8 mm, but they can get as large as 14.3 mm. Sizes above 10 mm in diameter account for only 5% of a pearl harvest.

     

    Nacre Coating

    A minimum of 0.8 mm to a 2.3 mm of nacre coating is expected after the 18-24 month culture period. Pearls with a coating below 0.8 mm are destroyed.

     

    Production

    Both Cultured and Mabe Pearls are produced. Production has been stable at less than 4 kilos per year, due mainly to problems caused by hurricanes but also by choice: High -quality production is attained with a controlled harvest. Mabe Pearl production is also limited to some 4-6 thousand per year.

    • 2004: 2.8 kilograms
    • 2005: 2.5 kilograms
    • 2006: 2.8 kilograms

    With an average weight of 1 gram per pearl, the yearly production is about 2,800 pearls.

     

    Treatments

    These pearls are untreated: no polishing, bleaching, irradiation, coating or artificial dyeing is performed on them. After harvest, pearls are soaked in water and then pat dried. The Sea of Cortez Pearl (or “Cortez Pearl”) is the only pearl in the gem industry that completely qualifies under the Fair Trade Gems protocols.

     

    Fluorescence

    The shell and pearls of the Pteria sterna oyster display a unique fluorescence under long-wave ultraviolet (UV) light, thus this pearl is able of displaying a blood-red to faint-pink glow. All other pearls (regardless of their origin) are inert (do not glow) under this same light.