Drusy: A crust of small crystals lining the sides of a cavity (or vug*) in a rock. Sometimes spelled druzy. Borrowed in the late 18th century from German druse, “weathered ore.”
According to an eHow entry on “drusy quartz”:
Drusy gems form naturally over millions of years. They are found on host materials such as agate or other minerals and are usually found when mining for other semiprecious gems, such as garnets and peridots. …
[Drusy] is recommended for pendants, brooches or earrings and does not hold up well to the wear and tear a ring or bracelet might receive.
Pendant with drusy and lapis lazuli from Bernadine Fine Art Jewelry.
Nevertheless, some jewelers do use drusy for bracelets:
Gold drusy bracelet from Etsy seller Frosted Willow.
And for rings:
Drusy jewelry has become popular only recently, according to the website of gemologist Barbara Smigel: “Until about 10 years ago, drusy minerals were little more than a curiosity, of interest to serious mineralogists, but unnoticed by jewelry designers, gem collectors, and the general public.” Drusy’s appeal, writes Spigel, “is easy to understand with its multitude of tiny crystals providing a reflective surface reminiscent of sugar or snow.”
Druse is also a botanical term referring to a rounded cluster of calcium oxalate crystals found in some plant cells. In ophthalmology, drusen (the German plural of druse) are tiny extracellular accumulations in the eye associated with aging and macular degeneration.
* One of my favorite Scrabble words.