Visit any gem and mineral show, and you will see crystals in many shapes and colors. But these are only a few of the thousands of different crystals that exist in the world. Encyclopedia Britannica writes, "... the term crystal is most often used by material scientists to refer to any solid with an ordered internal arrangement..." It takes special conditions for the kinds of crystals you see in rock shops to form.
What is a Crystal?
Like everything else in nature, crystals are formed by atoms. If a mineral has flat planes, known as crystal surfaces, and its atoms are grouped in regular patterns, then that mineral is a crystal.
All minerals belong to six crystal systems: cubic (pyrite), hexagonal (quartz), tetragonal (Zuecon), orthorhombic (topaz), monoclinic (malacite) and triclinic (axinite).
Chemical content and a wide range of environmental conditions determine what crystals grow in a specific area.
Crystals form beneath the Earth's surface. Igneous creation occurs when minerals crystallize from melting rocks. Metamorphic creation occurs when minerals form due to excessive pressure and heat. Sedimentary minerals form from erosion and sedimentation.
Water, temperature, pressure and good fortune all play a role in the creation of crystals.
As water travels over the Earth's surface, elements dissolve creating a nutrient-rich solution. If the water then evaporates, minerals are left behind. For example, if you have a bowl of sea water and let the water evaporate, salt crystals will be left behind.
If the water has a silica-rich solution, then quartz crystals such as amethyst may form. If the water is rich with copper, then malachite or turquoise will form.
Hydrothermal and Pegmatites
Hydrothermal deposits of minerals are created by either surface water making its way below ground or by cooling magma releasing water beneath the Earth's surface. The water finds a crack in the earth and crystals begin to grow. If the water comes from cooling magma, then tourmaline or emeralds may be found.
Sometimes low concentrations of water become concentrated as magma cools and solidifies to form a pegmatite. If the pegamite contains beryllium, beryl crystals will form. If they are rich in boron, tourmaline will form.
Magmatic, Metamorphic and Mantle
Magmatic gems, such as zircon, topaz and rubies, form in the holes in volcanic rocks. Others gems, such as jade, form when rocks are changed by heat, pressure and exposure to solutions. These gems are called metamorphic.
Olivine (peridot) is the most abundant of the mantle minerals. Diamonds are the most famous of the mantle minerals. Formed 100 miles beneath the Earth's surface, violent volcanic eruptions bring them to the surface intact.
Mine of Naica, Mexico
The mine of Naicia is rich with lead and silver. In order to reach these deposits, 18,000 gallons of 130 degree, nutrient-rich water is pumped out of an underground cave every minute. One thousand feet below the earth's surface grow the largest gypsum crystals in the world. Scientist believe that these crystals were formed by hytrothermal process.