Taaffeite

Crystal system Hexagonal
Transparency Transparent to Translucent
Luster Vitreous
Fracture Conchoidal
Cleavage
Specific Gravity 3.61
Hardness 8
Optical Character Uniaxial- Double Refractive
Refractive index 1.719 - 1.723
Birefringence 0.004
Dispersion 0.019
Fluorescence inert to week
green under U.V.
green under x-ray
Pleochroism
Chemical Formula MgBeAl4O8
Comments
Streak

This stone with the odd name taaffeite (pronounced TAR-fite) is another of the more recently discovered gem minerals, together with brazilianite, sinhalite and painite. It was named after Count Taaffe, an Irishman, who in 1945, noticed doubling of the back facet junctions in one light-violet stone that had been purchased as a spinel. Since he did not have instruments, other than a microscope, he sent it to the laboratory of the precious stone section of the London Chamber of Commerce, then headed by B. W. Anderson. There it was proved that the refractive index and specific gravity corresponded to no other mineral. The R.I. was 1.718 - 1.723, with a consequent birefringence of .004, and the S.G. 3.61; therefore, the confusion with spinel was understandable. Another stone, tested later by Mr. Anderson, had slightly lower indices: approximately 1.77 - 1.721. It has a hardness of 8, a vitreous luster, dispersion measuring .019, and is uniaxial negative. The chemical composition was determined to be a magnesium-beryllium aluminate (MgBeAl4O8), which is between spinel and chrysoberyl. The only other specimens of this rare mineral described to date were an 0.84 carat pear shaped stone, identified in 1958 by the Gem Trade Laboratory, the largest know, weighed 5.3 carats. Their properties correspond almost exactly with those of the first stone ever described.

Since the first taaffeite was found in a surplus stone box in Dublin, crystals of taaffeite have been reported in Australia and China. However, none of these have been of gem quality, and Sri Lanka is still assumed to be the source of gem taaffeites.

Since only about 10 gem taaffeites have been found, there is little reason to include taaffeite in a course on gemology, except for their curiosity value and because some gemologists have occasional opportunities to examine parcels of stones from Sri Lanka. Undoubtedly, more examples of this interesting stone will turn up from time to time.