Iolite: Gemstone Information
Though the name cordierite is used by mineralogists, the name iolite has become established among gemmologists so it is used (perhaps inconsistently!) here. Iolite is (Mg,Fe2)2Al4Si5O18 and forms short prismatic transparent crystals of the orthorhombic system and may be pseudo hexagonal. The cross-section is rectangular. Some crystals show twinning. Most gem iolite occurs as water-worn pebbles. Iolite has a hardness of 7–7.5 and SG 2.53–2.66, values increasing with higher iron content.The RI for the alpha, beta and gamma rays is 1.527–1.560, 1.532–1.574,1.537–1.578, biaxial positive or negative.Iolite is fashioned so that the blue colour is uppermost on a faceted stone or in the front of a statuette. The pleochroism is strong with violet-blue,pale blue and pale yellow to green colours. The iron absorption spectrum also varies with direction. Bands may be seen at 645, 593, 585, 535, 492, 456,436 and 426 nm. The absorption spectrum varies with the direction, so that in the direction of the violet-blue colour the 645 and 426 nm bands are masked by the general absorption and appear as cutoffs which shorten the spectrum.The variety bloodshot iolite found in Sri Lanka shows a red colour from oriented hematite platelets. The occasional four-rayed star stone turns up.Apart from alluvial, iolite occurs in a variety of environments, including altered aluminous and igneous rocks. Sri Lanka is the main source with some good crystals coming from Thompson, Manitoba, Canada.The gem gravels of Myanmar also produce iolite.