Gems formed from water near the Earth's surface

Water at or near the Earth's surface plays an important role in the formation of some minerals. The source of the water, in this case, is rain fall. Water can dissolve many minerals and carry away components such as silicon in solution. Silicon remains dissolved until the rainy season is over, the water evaporates, and silica minerals precipitate. Note that during the dry season, the water table falls, leaving behing deposits of silica minerals in cracks and open spaces (ametyst, agate, opal.

Malachite, a green copper-rich mineral, forms when acidic copper-bearing solutions react with limestone. Turquoise forms in a similar manner when acidic copper-bearing solutions seep into pores in volcanic rocks and react with aluminium and phosphorus.

Note that these are shown to develop by the same process as agate, amethyst, and opal, but the minerals occur in copper-rich host rocks.

Agate and amethyst can also be formed hydrothermally (from hot water - the water is heated by magma intrusions, as we will see in the next environment).

In all the above gem-forming reactions, the key things to note are:

- a source of elements (the rocks at the Earth's surface

- a rainy season, when the rocks dissolve

- a dry season, when the solutions precipitate minerals (year after year).

Note that most opals, for example, are found in desert regions, where this type of climate pattern is most pronounced!