|North America||Beaver, Iron County, Utah, USA||2 ¼" x 1 ½" x 1 ¼"|
The purple color of amethyst (which is simply purple quartz) is caused by a color center in concert with radiation. This particular specimen has a very subdued color. That likely means either that there is very little iron in it, or that there was very little natural radiation produced by the rocks in which it formed (see Learn More under Specimen #4 for a description of the cause of color in amethyst).
The flat sides of these crystals are called crystal faces. Because these particular crystal faces are flat and lustrous, it is clear that the crystals grew into an unimpeded environment, such as a cavity in the surrounding rock. The combination and relative sizes of faces on a crystal constitute what is called its "habit" – the typical shape in which it occurs. This particular habit - long compared to its breadth - is known as prismatic, and the "points" on the ends are known as the terminations. So these are prisms terminated by pyramids. Some of the crystals are terminated on both ends (doubly-terminated), which is unusual for any variety of quartz, as quartz crystals typically grow out from a surface or wall of a void in the rocks and show only one termination.
Many minerals occur in only a few typical habits, and other minerals display many different habits. The reasons for the same mineral having different habits are complex and not entirely understood for specific cases, but the temperature, pressure, and chemical environments surely play key roles.