Here is a Question from One of Our Customers:

Q:  What would be the best way to define a crystal? What is the definition in other words?

Answer:

Dear S,

All crystals are one or more minerals. Crystals need three things to grow: space, mineral soup and heat.

So here is the dictionary version of a crystal.
a. A homogenous solid formed by a repeating, three-dimensional pattern of atoms, ions, or molecules and having fixed distances between constituent parts.
b. The unit cell of such a pattern.
2. A mineral, especially a transparent form of quartz, having a crystalline structure, often characterized by external planar faces.
http://www.thefreedictionary.com/crystal

Roslyn Bohanan