Taaffeite

Key Differentiators

• Uniaxial negative — THIS separates it from spinel (isotropic, SR)
• RI 1.719–1.723 — similar to spinel but taaffeite is doubly refractive
• SG 3.61 — slightly lower than spinel (3.52–3.78 range)
• Historically mistaken for spinel before identification (polariscope is definitive)
• One of the rarest gem minerals; Sri Lanka, Tanzania, and China

Common Simulants

• Spinel: Taaffeite was first identified by confusing it with spinel — the two are closely related. Spinel is isotropic (no birefringence); taaffeite is doubly refractive (birefringence 0.004, uniaxial negative). Use a polariscope or refractometer: spinel RI ~1.718 (single reading); taaffeite RI 1.719–1.723 with a double reading.
• Pink Sapphire: Pink sapphire (corundum) has similar transparency and color range in pink/lavender hues. RI 1.762–1.778 (much higher than taaffeite 1.719–1.723); SG ~4.00 vs taaffeite ~3.61. Hardness 9 vs taaffeite 8–8.5. Refractometer clearly distinguishes.
• Amethyst: Purple/lavender amethyst may resemble pale taaffeite in these colors. Much lower RI 1.544–1.553 (vs taaffeite 1.719–1.723) and SG 2.65 (vs taaffeite ~3.61). Easily distinguished by refractometer; amethyst is far lighter when hefted.

About Taaffeite

Taaffeite (BeMgAl₄O₈) was first identified as a new mineral species in 1945 after Count Taafe purchased a faceted stone from a Dublin jeweler assuming it was spinel. The polariscope immediately separates them: spinel is isotropic (SR), taaffeite is uniaxial negative (DR). RI is similar to spinel (1.718) but taaffeite shows weak double refraction. One of the rarest gem minerals; a few hundred gem-quality specimens are known. Most material is from Sri Lanka and Tanzania; a small deposit in China (Inner Mongolia) was documented in the 1980s.