Word of the day: Botryoidal
This word is derived from the Greek botrus [bunch of grapes], and is used to refer to any mineral that has formed in globular, rounded blobs, which could be said to look somewhat like grapes.
Botryoidal is one of many crystal habits it is possible for minerals to form in, and some minerals are more commonly found in this shape than others. Hematite is commonly found in the botryoidal crystal habit, as is fluorite, goethite, and malachite, among other minerals. Nephrite jade also forms in botryoidal masses sometimes, but it is rare. Good quality botryoidal jade can be very valuable, so if you find some, you're in luck.
And, of course, some mineral collectors base their entire collections around a single crystal habit, so any mineral that was only rarely found in botryoidal form could have a great deal of value to the right person.
The botryoidal habit is caused by hundreds of very tiny, slender crystals growing from a central point for each rounded mass. The tops of the crystals are smooth and blend with each other, so individual crystal edges are not noticeable - unless you break one open, and then you can usually see the crystals radiating outwards. But please don't go smashing lovely botryoidal specimens just to look at it!
Reniform and mammillary crystal habits are basically the same thing as botryoidal, all that really differs is the size of the blobs. Reniform is the largest, with mammillary in the middle and botryoidal at the small end. However, most people who aren't mineralogists just use botryoidal to describe them all.
Got all that?
Great! Get out there and find some botryoidal minerals, then.