Aragonite: Gemstone Information
The orthorhombic polymorph of CaCO3, aragonite, has been found in transparent colourless, yellowish, yellowish green and brownish yellow crystals at Horuenec near Bilina in the former Czechoslovakia. Aragonite was named after the original locality in Aragon province, Spain and has a hardness of 3.5 to 4 on Mohs’ scale. The SG is 2.93–2.94 and the RIs are 1.530–1.540; 1.680–1.690 with a birefringence of 0.150; the optic sign is negative. An opaque, iridescent aragonite of ammonite shells comes from a location between Cardstone and Lethbridge in Alberta, Canada.RI’s are 1.522, 1.672–1.673 and 1.676–1.679 with a birefringence of 0.154–0.155. This material, known by the trade name of ammolite or korite, will be discussed below. Aragonite frequently shows pseudo-hexagonal habit which is caused by combination and twinning. As a carbonate mineral, it is easily soluble in acids.