Shop Hyalite Opal I Idocrase Refractive Index 1.7 - 1.723 Double Refraction Yes Birefringence 0.002 to 0.012 Idocrase Information Shop Idocrase Imperial Topaz Refractive Index 1.609 - 1.643 Double Refraction - Birefringence 0.008 to 0.016 Imperial Topaz Information Shop Imperial Topaz Iolite Refractive ...