They are also used in jewelry, such as diamonds. The Merriam-Webster's Collegiate Dictionary defines a crystal as quartz that is either transparent or nearly so and that is either colorless or slightly tinged. Crystal Formation Crystals form whenever a solid is formed from fluid. Crystals form from vapors, solutions or molten materials, and are built from repeating units. Crystals grow from the outside, salt crystals build up from sodium and chloride ions.
Snowflakes form from water vapor.
Rocks are usually made up of 1 or more minerals. Most non-living substances are made of crystals. If the cooling of obsidian is slower than usual, a new rock is formed called felsite. Felsite is crystalline, and its crystals cannot be seen with the naked eye. There are many different shapes and kinds of crystals.