Hambergite is one of the lesser-known gemstones. It is usually nearly colorless, with the vitreous luster of glass when cut. It is quite a hard material, with a hardness of 7.5 on the Mohs scale, so it takes a very good polish.
Hambergite derives 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 is easily visible by looking down through the table of a Hambergite specimen -- you will see double images of the back facets due to the strong double refraction.
By chemical composition, Hambergite is a beryllium hydroxyborate. It has excellent hardness (7.5) and 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 low density mineral 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 colors range from completely colorless to gray-white and yellow-white. It may also appear as nearly colorless with a slight pink or purple tint.
Madagascar is the best know 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).