All of these terms are misleading and probably relate to incidents where shifty characters attempted to pass off rock crystals as more valuable diamonds to unsuspecting investors. At one time it was believed that rock crystal was ice that had become so frozen it could no longer melt. Throughout the ages magicians and astrologers have used rock crystals as a gazing globes in which they can see the future and visit with apparitions beyond the veil. Dendritic agates are similar, but have moss-like, or denritic, inclusions trapped in clear to translucent chalcedony. Both are favorites with jewelry designers.
Amethyst is perhaps the best known and most admired variety of quartz.
The ancient Greeks believed that the wearer of an amethyst jewel would be unaffected no matter how much wine they drank. Set in yellow gold amethyst also makes for a wonderful jewel. The finest amethysts display a purple to purplish-red hue with flashes of violet and red. Since it is an alternative birthstone for November, citrine is often used as a substitute for more expensive topaz. Few people realize that many citrine specimens started out as smoky quartz or amethyst crystals that were then heat treated.