Chrysoberyl forms as a result of
pegmatitic processes. Melting in the
Earth's crust produces relatively low-density molten
magma which can rise upwards towards the surface. As the main magma body cools, water originally present in low concentrations became more concentrated in the molten rock because it could not be incorporated into the
crystallization of solid minerals. The remnant magma thus becomes richer in water, and also in rare elements that similarly do not fit in the crystal structures of major rock-forming minerals. The water extends the temperature range downwards before the magma becomes completely solid, allowing concentration of rare elements to proceed so far that they produce their own distinctive minerals. The resulting rock is igneous in appearance but formed at a low temperature from a water-rich melt, with large crystals of the common minerals such as
quartz and
feldspar, but also with elevated concentrations of rare elements such as beryllium,
lithium, or
niobium, often forming their own minerals; this is called a
pegmatite. The high water content of the magma made it possible for the crystals to grow quickly, so pegmatite crystals are often quite large, which increases the likelihood of gem specimens forming. Chrysoberyl can also grow in the
country rocks near to pegmatites, when Be- and Al-rich fluids from the pegmatite react with surrounding minerals. Hence, it can be found in
mica schists and in contact with metamorphic deposits of
dolomitic marble. Because it is a hard, dense mineral that is resistant to chemical alteration, it can be weathered out of rocks and deposited in river sands and gravels in alluvial deposits with other gem minerals such as diamond, corundum, topaz,
spinel,
garnet, and tourmaline. When found in such
placers, it will have rounded edges instead of sharp, wedge-shape forms. Much of the chrysoberyl mined in
Brazil and
Sri Lanka is recovered from placers, as the host rocks have been intensely weathered and eroded. If the pegmatite fluid is rich in beryllium, crystals of beryl or chrysoberyl could form. Beryl has a high ratio of beryllium to aluminium, while the opposite is true for chrysoberyl. Both are stable with the common mineral quartz. For alexandrite to form, some
chromium would also have had to be present. However, beryllium and chromium do not tend to occur in the same types of rock. Chromium is most common in
mafic and
ultramafic rocks in which beryllium is extremely rare. Beryllium becomes concentrated in
felsic pegmatites in which chromium is almost absent. Therefore, the only situation where an alexandrite can grow is when Be-rich pegmatitic fluids react with Cr-rich country rock. This unusual requirement explains the rarity of this chrysoberyl variety. ==Alexandrite==