ICELAND SPAR

2 clear colorle variety of calcite (CaCO3) found in Iceland. It forms large rhombohedra having a specific gravity of 2°7 and a hardne =3. The value of I. S. lies in its having a strong double refraction, which makes it pre-eminently suited for polariscopes, Nicot’s prisms, and other optical purposes. The supply from Iceland, where crystals of very large size are found, is nearly exhausted, and no substitute has been found to compare with it.