You see everyone using them. Decorating their homes, offices, classrooms, studios, their bodies, and even incorporating these sparkly bits of rock in their cosmertics and skin care. But what exactly are crystals?
I guess the best way to define crystal is a mineral or combination of minerals that have formed within an extremely well organized molecular structure. (Think of the playground cube monkey bars from your childhood.)
Now, not all crystals are going to take the same shape as another type of crystal, but most of them will have an organized molecular set up that gives them a distinct look that is easily recognizable by those who are familiar with them. Sometimes though, environmental factors such as water and wind can alter their natural forms and make them somewhat unrecognizable even to those who are trained to know what to look for.
I guess the next question would be how are they formed? Well that's an easy one. The molecules of the liquified minerals (whether it be via a water solution, or molten via the high temperatures of the inner layer of the earth), are floating along about their business, until certain environmental changes cause them to start to solidify. The process of them solidifying can be a as slow as millions of years or, it can be as quick as a few days.
Each crystal, as I stated earlier, will form a bit differently according to their molecular makeup and environment. Diamonds for example, one of the most popular (and most expensive) crystals out there, forms under extreme pressure and extreme heat from carbon. Yup. That black stuff that forms on your pans or grills after stuff has been cooking way too long. That burnt black material may formerly be food, but it was reduced under those high temperatures to carbon. Makes you look at that brightly twinkling rock on your hand a bit differently, doesn't it?
Not all crystals will have a highly organized molecular structure. Some will come out amorphous. Examples of this are turquoise, opals, malachite, chrysocolla, etc. But it still doesn't change the fact that they are crystals.