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Structure and Shape
Crystals take on a special structure, known as a crystalline lattice, at the microscopic level as they are formed, according to the MadSci Network. This is why most crystals feature flat sides and a rectangular or trapezoidal shape, instead of the rounded or random shape of a regular rock. Identifying a crystal by its shape becomes difficult when it is surrounded by another type of mineral or a larger growth of rock. Geodes are round and smooth on the outside and may be confused with normal rocks. When they are broken open, the crystals inside are easily identified by their regular shape. Points often develop on one end of the crystal due to the design of the crystalline lattice.
Cooling Speed
All hardened mineral deposits are formed by heat, high pressure or a combination of the two. For a crystal to form, the minerals that compose it must cool slowly, says the Michigan State University Ask Science Theatre website. Slow cooling processes give the molecules in the crystal time to settle in the special lattice structure that gives crystals their shape. Crystals embedded in larger pieces of rock, like the tiny structures in a slab of granite, form because the outside of the rock cools quickly but the heat inside is trapped and dissipates much more slowly. Slow cooling makes some minerals very hard, while others become softened or brittle. Diamonds are a very hard type of crystal, but smaller pieces of selenite can be broken by hand.
Purity
Many minerals make up the average rock. Due to the variations in the size and shape of each mineral's molecules, mixed minerals cannot settle into the ordered and regular structure of a crystal, according to the Utah Education Network. This creates the different shapes and textures of rocks. All specimens of granite share similar characteristics, but the exact amount of each mineral and the variations in the cooling process create different colors and patterns in the material. Two pieces of a rock can vary so much that it takes a chemical test to verify their composition. In most cases, crystals feature the same colors, structures or other identifying characteristics due to the high purity of the mineral from which they are formed.
Evaporation
When water runs over a mineral deposit, it may dissolve and pick up small particles of the material. Deposits form when the water evaporates, and crystals may develop in these deposits. The MadSci Network website says that stalactites and stalagmites are large crystals that have formed this way. Rocks, on the other hand, are only formed through pressure or heat. The crystals formed through evaporation may not feature the exact shape that heat- or pressure-formed crystals do, but they share other characteristics such as mineral purity and a regular microscopic structure.