A deep violet or black colored quartz crystal is usually termed as black amethyst. It is commonly used for jewelry making procedures. It is reputed to control bad thoughts and accordingly, travelers used to wear this stone jewelry as a protective covering from danger. Black amethyst may also refer to black amethyst glass. The glass has been around since the late 19th century, and commonly takes the shape of bowls or vases.  Black amethyst appears black, but it will look like a dark purple when held to light.