According to The Coffee Detective: Coffee trees produce red or purple cherries which typically contain two seeds. Open the fruit and you’ll find two green coffee beans, face to face.
When the coffee cherries are harvested, between 5% to 10% of the cherries will contain only one bean. Coffee cherries containing just one bean are referred to as peaberries.
According to Wikipedia Peaberry, known in Spanish as caracolillo, is a type of coffee bean. Normally the fruit (“cherry”) of the coffee plant contains two seeds (“beans”) that develop with flattened facing sides, but sometimes only one of the two seeds is fertilized, and the single seed develops with nothing to flatten it. This oval (or pea-shaped) bean is known as peaberry. Typically around 5% of all coffee beans harvested have experienced this small mutation.
Peaberry coffees are particularly associated with Tanzanian Coffee, Check out Zawadee Spotted Cat Tanzanian Peaberry although the peaberry variety of Kona coffee Check out Koa Coffee Kona Peaberry has also become quite prominent.