The main difference between glass and crystal is the lead content. Glass is made of sodium and potassium, while Lead crystal is made of silica sand, lead oxide, and potassium carbonate.  These raw materials are combined in furnaces that reach temperatures of 2500-3000 degrees Fahrenheit.  The addition of the lead oxide changes the refraction index on the glass, and the highest quality pieces will be clean, bright, and have the most brilliance.

Because lead crystal is softer than, and not as brittle as standard glass, it is possible to make deeper and more complicated cuts in crystal, lending itself to the traditional cut patterns that enhance the light refraction on the glass.  A heavily cut piece of crystal, like a diamond, will reflect the most light and prisms of color.