(via ICA Early Warning Flash, No. 36, April 2, 1990) GII writes:
Subject: Beads of Tourmaline-Spinel Doublets in a string of natural rubies.
Observations: The string consisted of 52 beads of red color. Each bead (average weight 20 carats) was individually tested for its gemological properties. 50 pieces were identified as Natural Rubies having inclusions of rutile and some crystal inclusions. All beads were transparent red colored and irregular in shape. All beads were showing dichroism in some position or the other.
Microscopic examination of the beads revealed that two of the beads were actually doublets. One portion of these two beads had typical inclusions of spinel (octahedral crystals and thin liquid films) and the other portion showed characteristic inclusions of tourmaline (thread-like cavities and trachites). On further examination the line of demarcation could also be noticed. Refractive indices (spot method) clearly showed that the beads were doublets of red tourmaline and red spinel; R.I 1.62 and 1.719 for tourmaline and spinel respectively.
Warning: The red colored doublets are beads of irregular shape and can be easily mistaken for rubies in the necklace. It is not sufficient to test only a few pieces in the necklace. The refractive indices for the beads should be obtained from different directions. It is also imperative to check the inclusions for all the beads in a necklace.
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