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Crystal is composed of molecules, ions and atoms which are properly ordered to form a transparent internal structure. There are three most common types of crystals which are often used for experiments or you can do grow the at home; Alum Crystals, Salt Crystals and Sugar Crystals.

Growing Salt Crystals
Step 1
Boil 4 cups of tap water and add plenty of salt once the water starts to simmer so that by the time it will boil, no salt will be left un-dissolved.
Step 2
Put a generous amount of salt solution into the saucer and let it evaporate so that what will be left are the salt crystals.
Step 3
Select the one you want to grow and tie it with nylon.
Step 4
Transfer the salt solution into a clean jar and hang the crystal you tied with nylon to submerge. You can place them in a place where they will not be disturbed.
Step 5
Let them be for about a week or until the salt crystal cease to grow. Removed it from the jar and let it dry.

For other types of crystals see the following guides:

How to Grow Sugar Crystals

How to Grow Alum Crystals