Throughout the world, the coffee fruit occasionally produces a single, rather than a double, bean. It grows to be small and round, with a tiny crevice that splits it halfway down the middle. Called peaberry in English and caracol in Spanish, these beans are often separated from normal-shaped beans and sold as a separate grade of the same coffee.
According to coffee folklore, peaberry grades are considered superior to normal grades from the same crop, apparently on the basis that, in peaberries, the good stuff that ordinarily goes into a double bean goes into only one bean. I am not sure peaberry tastes better than normal beans from the same crop, but it does taste different. Typically, peaberry is more buoyant and more brightly acidy, more complex in the upper aromatic ranges of the profile but somewhat lighter in body, than comparable normally shaped beans.
Peaberry coffee should be sold by country and market or estate name like any other coffee. If you read a notice that simply says peaberry, you should inquire about the origin of the coffee.