Interestingly enough, emerald and various other varieties of beryl seldom occur in the same kinds of deposits, although their manner of formation is similar. With the exception of emerald, all of the varieties tend to be associated with pegmatite dikes as the most important single source for these various colors. Emerald, on the other hand is found either in schistose rocks or as one of a number of minerals filling veins in other rocks; however, the mineralizing solutions or gases were magmatic in origin and therefore like pegmatites.
Emeralds in the important Colombian mines at Muzo occur in calcite veins that traverse both a sooty shale and a rather thin bedded carboniferous black lime stone. Despite their presence in a bedded sedimentary rock, the origin of the solutions that deposited the emerald crystals and calcite in the veins was pegmatitic in origin.
Below the
formation in which the emeralds are found is a rock composed largely
of albite feldspar. The rock which became albitized by the
introduction of solutions from some deeply buried magma, is formed by a
type of metamorphism that takes place at rather greater distances from
the molten rock than would justify the usual term "contact metamorphism". Investigators of the area have remarked on the nature of the albite
and its somewhat transparent nature and drusy concentration at places in
the rock. It appears that the albite replaced the calcite in the
formation.
An adjoining rock contains large calcite rhombs. Undoubtedly, the solutions dissolved calcite from the rocks and re-deposited it in the
veins. Also present are such evidences of the nature of the mineralizing
solutions as fluorite and apatite.
In Russia, at Habachtal in the Austrian Alps, is South Africa, in Rhodesia, and in one Brazilian area the emeralds are found as a constituent of mica-schists. Emerald is one of the few gems that is not found in quantity in alluvial deposits; it is almost the only important gemstone that is mined exclusively from primary sources, the rock in which it occurs. The green beryl found in Brazil is a constituent in a dolomitic marble of the contact-metamorphic type. Although most gem occurrences of beryls other than emerald are in pegmatite dikes, they also occur in other kinds of deposits.
On occasion, aquamarine is found in mica-and talc schists and quartz veins. In Russia, it has been found associated with topaz and amethyst in a topaz-quartz rock cutting across granite. Most of the gem material, however, is found in pegmatite dikes and in most of the gem bearing pegmatites of the world. Beryl is much like topaz and tourmaline, in that it forms either as a vein mineral or in pegmatite dikes rather late in the cooling of a molten mass when the volatile constituents have become concentrated by the crystallization of the other ingredients. They may then be concentrated in dikes or pass out through fractures in the rocks, forming veins.
Since in its pure form beryl is colorless, the different varieties are colored by the presence of minute amounts of metallic oxides, other than those that are essential constituents of the mineral. When chromium oxide is present, the usual result is emerald, whereas aquamarine is thought to have been colored by ferrous iron, morganite by lithium and golden beryl by ferric iron.
- Varieties of Beryl
- Formation of Beryl
- Sources of Emerald, Beryls
- Polishing Emeralds
- Physical & Optical Properties of Emeralds
- Test and Identification of Emeralds
