Hambergite is one of the lesser-known gemstones. It is usually almost colorless, with a vitreous (glassy) luster when cut and polished. It is quite a hard material, with a hardness of 7.5 on the Mohs scale, so it is quite a hard material. However, it has perfect cleavage and a brittle tenacity, which makes it quite fragile and susceptible to fracture.
Hambergite gets its slightly curious name from the Swedish geologist and geographer Axel Hamberg (1863-1933). Hamberg taught crystallography and mineralogy at Stockholm University and then became professor of geography at Upsala University. From 1913 to 1927 he was president of the International Iceberg Commission.
There are several features that make hambergite quite unique. It has very strong double refraction, with a birefringence rating of 0.072, much higher even than zircon. This property can be observed when looking down through the table of a hambergite specimen - you will see doubling of the back facets due to the strong double refraction.
By chemical composition, hambergite is beryllium hydroxyborate. It has quite a low density of 2.35 (about the same as obsidian). Hambergite has a refractive index of 1.553 to 1.628, a rather wide range due to its pronounced birefringence. It is unusual for a mineral with a low density to have such a high refractive index, since most gems with a high refractive index, such as corundum, zircon and garnet, are also very dense materials.
Hambergite from Madagascar
Hambergite can be completely colorless, gray-white or yellow-white. It may also appear almost colorless with a slight pink or purple tint.
Madagascar is the best known contemporary source for hambergite, though it remains a rare gem, especially in larger sizes. Other deposits are found in Burma, Pakistan, Afghanistan, Tajikistan and in the United States (California).