Well first if it is crystal it is a mineral in a rock matrix. If it's clear or light in color it's probably quartz crystal. There is Smokey quartz, rosey quartz and the clear.Quartz usually forms 6 sided prisms. It is a very common mineral and it's made of oxygen and silicon. They are pretty to look at but have little value. I could go on and on about cleavage etc. but I hope I gave you what you want to know. Yep, I'm a rock hound.
You can tell if a crystal is real by examining it for scratches. Press it hard against a glass surface and check for scratches. If it has scratches, then it is not real. Check the edges of the crystal to make sure that they are hard. Visit http://www.ehow.com/how_5753956_tell-quartz-crystals-real.html for details.