are green and black olives from the same tree
The fruit is a small drupe 1--2.5 cm (0.39--0.98 in) long, thinner-fleshed and smaller in wild plants than in orchard cultivars. Olives are harvested in the green to purple stage. Canned black olives have often been artificially blackened (see below on processing) and may contain the chemical ferrous gluconate to improve the appearance. Olea europaea contains a seed commonly referred to in American English as a pit or a rock, and in British English as a stone.
Olive