Spring 2010

Featured Gem News International: Diopside from Ihosy, Madagascar

Two faceted samples, two well-formed prismatic crystals, and several small pieces of rough were loaned to GIA for gemological observation (e.g., figure 1). Examination of the two cut stones (5.93 and 6.50 ct) and some of the rough pieces showed the following properties: color—medium- dark yellowish green; pleochroism—very weak; RI—1.676–1.700; birefringence—0.024; hydrostatic SG—3.27–3.30; no reaction to the Chelsea filter; inert to long- and short-wave UV radiation; and absorption lines at 490 and 505 nm visible with the desk-model spectroscope. These properties were consistent with diopside (R. Webster, Gems, 5th ed., rev. by P. G. Read, Butterworth-Heinemann, Oxford, UK, 1994, pp. 330–331).

The two faceted samples contained only minor inclusions, consisting of a few “fingerprints” and some small fractures. Iron-stained fractures and strings of pinpoints were visible in the rough material. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis of one faceted sample showed major amounts of Si, Ca, and Mg, and minor Fe—as expected for diopside (CaMgSi₂O₆), which forms a series with hedenbergite (CaFeSi₂O₆). The relatively low RI values are consistent with a near–end member diopside composition (see W. A. Deer et al., Rock-Forming Minerals—Single-Chain Silicates, Vol. 2A, 2nd ed., John Wiley & Sons, New York, 1978, pp. 198–293).

Although gem-quality diopside from Madagascar has been reported previously in the literature (e.g., U. Henn and C. Milisenda, “The gemstone occurrences of Madagascar,” Australian Gemmologist, Vol. 21, 2001, pp. 76–82), to our knowledge this is the first description of this material.

Riccardo Befi
GIA Laboratory, New York

Note: A description of the faceting of these stones is available here.