Elahera brown dravite tourmaline: trigonal crystal pleochroism and specific gravity?
What optical absorption lines and dichroscope colors verify high-magnesium dravite tourmaline from brown zircon in Central Province alluvial washings?
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5 Answers from travellers
Brown dravite tourmaline from the Elahera gem field is a complex sodium-magnesium borosilicate occurring in metasedimentary marbles and pegmatite skarns. It is trigonal, exhibiting strong dichroism under a calcite dichroscope: dark yellow-brown parallel to the optic c-axis and pale honey-yellow perpendicular to the axis. It has a refractive index of 1.624 to 1.650 and specific gravity of 3.03 to 3.10. Brown zircon occurring in the same gravels is easily distinguished by its higher specific gravity (4.6-4.7) and strong birefringence lines (0.059) that double pavilion facet edges.
Lapidarists must orient the table facet parallel to the c-axis; looking directly down the c-axis reveals extreme optical absorption that renders stones opaque black.
Dravite crystals feature rounded triangular cross-sections with heavy vertical striations along prism faces, distinct from hexagonal beryl and corundum.
Heat treatment of brown dravite at 450°C can lighten dark overtones into bright golden-yellow, but must be disclosed on NGJA gem certificates.
Elahera rough gravels are washed in river sluice cradles; tourmaline pebbles display oily luster and characteristic basal conchoidal fracture scars.
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