ABSTRACT
Specific gravity: 2.68-2.78 (2.83-2.86 for conch pearls) Hardness: 2.5-3.5 Luminescence: Most black pearls fluoresce a faint reddish brown under LW UV Occurrence: The Kuwait-Bahrain Gulf, the Gulf of Manaar, the South Sea Islands,
the Pacific coast of Mexico (black pearls), the Gulf of California (black pearls), the north-west coast of Australia, and around the islands and inlets of Japan. Pink (non-nacreous) conch pearls are found off the coasts of Florida and the West Indies. Fresh-water pearls from the rivers of the UK, the USA, the former USSR and the People’s Republic of China
Shell Composition: Overlapping layers of submicroscopic crystals of calcium carbonate
forming the inner nacreous layer of a mollusc’s shell Varieties: Mother-of-pearl from various shells having an iridescent lustre including
Pinctada maxima, Pinctada margaritifera and Haliotis (also called ‘abalone’, ‘paua’ or ‘ear shell’). The shell of the turbo sea snail is used to produce imitation pearls (called ‘antilles’ or ‘oil’ pearls). Cameos are cut from shells having layers of contrasting colour or shade, such as the helmet shell (Cassis madagascariensis) and the shell of the giant conch (Strombus gigas)
Occurrence: The Pinctada and Haliotis shells are found in American, Australian, Japanese, Korean and New Zealand waters. The helmet shell is found in West Indian waters, and the giant conch shell in both West Indian and Florida coast waters
Tortoiseshell Composition: Derived mainly from the shell of the Hawksbill sea turtle, this material
consists mostly of keratin (a protein forming the basis of horns, claws, nails, etc.) Refractive index: 1.55 Specific gravity: 1.29 Hardness: 2.5 Luminescence: The yellowish parts of the shell fluoresce a bluish white under LW UV Occurrence: The Hawksbill turtle is found in tropical and subtropical seas,
particularly around Malaysia, the West Indies and Brazil
Chemical composition, varieties, constants, characteristics and occurrence
If with doubly refracting stones the difference between the two Rls is significantly greater than the figure for DR, this indicates the possible range in which RI values can occur.
