If you link to the following pages, then you can download the software to do crystal projections and find other interesting things on your own. An example of such a crystal structure is illustrated on the left. So, the unit cell is the basic repeat-unit for describing a crystal. Well most of us have seen mineral crystals. For example, consider amethyst.
Amethyst is the purple variety of quartz and is a popular gemstone.
If it were not for its widespread availability, amethyst would be very expensive. This was maybe due to a belief that amethyst would ward off the effects of alcohol, but most likely the Greeks were referring to the almost wine-like color of some stones that they may have encountered. Its color is unparalleled, and even other, more expensive purple gemstones are often compared to its color and beauty. The reference URL for this image is a mineral supplier . The amethyst crystals, above-left, are large and well defined.