A coffee bean is a seed of the Coffea plant and the source for coffee. It is the pip inside the red or purple fruit often referred to as a cherry. Just like ordinary cherries, the coffee fruit is also a so-called stone fruit. Even though the coffee beans are not technically beans, they are referred to as such because of their resemblance to true beans. The fruits; cherries or berries, most commonly contain two stones with their flat sides together. A small percentage of cherries contain a single seed, instead of the usual two. This is called a "peaberry". The peaberry occurs only between 10% and 15% of the time, and it is a fairly common (yet scientifically unproven) belief that they have more flavour than normal coffee beans. Like Brazil nuts (a seed) and white rice, coffee beans consist mostly of endosperm.The two most economically important varieties of coffee plant are the Arabica and the Robusta; ~60% of the coffee produced worldwide is Arabica and ~40% is Robusta. Arabica beans consist of 0.8–1.4% caffeine and Robusta beans consist of 1.7–4.0% caffeine.