What is Crystallisation?
Crystallisation which can be a natural or artificial process is the formation of a solid due to the build up crystals from a homogeneous solution. The word homogeneous comes from Greek and means 'same kind'. A perfect example is water freezing to form ice or snow. When atoms come together in an orderly repeating pattern they form a crystal.
Materials may also solidify as they precipitate out of a liquid or gas. Precipitation in mineralogy refers to a dissolved mineral that comes out of water, an example could be a teaspoon of salt which has previously been dissolved in a glass of water.
Crystallisation can take place in nature very quickly or it can also take place over thousands of years. Natural crystals from deep within the Earth's crust took millions of years to form. Crystals grown at home can form in anything from a few hours to a few days.
Place three cups of sugar or salt in one cup of boiling water and stir until dissolved. Transfer the solution to a jar and drop in a wooden skewer or better still a piece of string that has been roughened with a knife. Crystals grow better on a rough surface than on one that's smooth. After a few days as the water begins to evaporate crystals will form (precipitation) and you'll end up with a mass of crystals. That's crystallisation.
If the saturated solution cools too quickly you'll get smaller crystals, cool it slowly and the crystals will have more time to grow. The process is exactly the same in nature. Granite is known for its large crystals because the molton rock cooled very slowly over millions of years deep beneath Earth's surface. Obsidian which is natural volcanic glass is known as a mineraloid which means it has no significant crystal structure. Obsidian forms when magma erupts from the vent of a volcano and cools so quickly that crystals don't have time to grow.
If you want to see the effects of crystallisation and are in a hurry, just place some water in the freezer and within a few hours you'll have ice which is crystallised water.