Gemstones are formed in the Earth's crust. Basically, crystals are fossilized water. They (gemstones and minerals) are usually composed with compounds comprising with more then one element. These compounds in the right conditions (in the earth's crust) of high temperatures, energy and pressure come together, and when everything is just right, they cool to form (grow) into crystals, gemstones and minerals.
Sand and silicon, combines with water and becomes quartz crystal.
Crystals continue to grow until all the free atoms have been arranged or until the conditions change.
The higher the temperature and pressure the harder the crystal - like Diamond, Emerald and Quartz. The lower the temperature and pressure softer minerals occur like - Calcite, Turquoise and Selenite.