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What Are Crystals?
A crystal is a hard, solid substance made of molecules that bond together in specific patterns to form an interesting shape that has straight edges and flat surfaces. Not all crystals have the same shape, as you can see in the salt vs. sugar project - there are actually lots of different kinds of crystals, and each kind has its own special shape. Some crystals that you are probably very familiar with are sugar, salt, and ice. But many other solids are made of crystals too; we just can't see them because they are so small! Lots of minerals form beautiful crystals that are used for jewelry, like diamonds or emeralds. Crystals are often transparent, which means that you can see through them sort of like you can see through glass. Other kinds have beautiful colors.
What Are They Made Of?
What a crystal is made of actually depends on what kind of crystal it is -- for example, salt, sugar, and snowflakes are actually formed out of different kinds of crystals! Snowflakes are made from crystals formed by frozen water. (You can see a picture here.) Salt crystals are formed by some chemical elements -- sodium and chlorine -- which join together in a crystal shape. (You can see a picture of salt crystals here.) Sugar crystals and rock crystals are made up of different chemical elements, too.
Crystals can be formed in several different ways. Most crystals are formed through evaporation. For example, when water from saltwater evaporates (or is dried up into the air), salt crystals are left behind (do the Borax Snowflakes project to see this happen). Ice crystals are formed when water from the Earth evaporates into the air and becomes a gas called water vapor. The water vapor becomes clouds and then freezes and falls back down to earth as snow. Ice crystals can also form as frost on windows and on the ground when the air has a lot of moisture (water vapor) and the temperature is below freezing. Some types of crystals are formed from melted rock in the earth (remember this from when we talked about Volcanoes?). When the hot rock cools gradually, it will sometimes form crystals. Geodes are round rocks that are formed when bubbles are trapped in the melted rock. As the bubbles cool down, crystals grow inside of the bubble of rock!
As a crystal grows, the pattern that makes it a certain shape will be repeated over and over, so the crystal will always keep the same shape as it gets bigger! The chemical elements that a crystal is made of are what tell the crystal what shape it will be. A crystal of salt is a different shape than a crystal of sugar (do the Salt vs. Sugar project to see for yourself!) because they are both formed from different elements. A lot of crystals might seem to look alike at first glance, but what elements the crystal is made out of will make it a unique shape and color. Even the same element can make different crystals, though, based on conditions such as temperature and light and what other elements are around. For example, the graphite used inside of pencils is a kind of crystal made from the element carbon, which is actually the same element that diamonds are formed from!
Ways Crystals Are Used