Have you ever wondered how many species of Olive trees there are in the world? It is a natural question to ask given the wide variety of Olives that can be purchased at the grocery store. Well, the answer is ONE. There is just one species of Olive tree "Olea Europaea".
One common assumption is that black olives grow on one species of Olive tree and green olives grow on another species of Olive tree. This, however, is a misunderstanding of the difference between black and green olives. The difference in color is a result of when they are harvested and in fact all olives will eventually turn a dark red/purple or almost black color if left to full ripen on the tree. Green olives are harvested once they have reached their maximum size but before they begin to turn dark purple. The best time to harvest is when the color begins to turn from a green color to a lighter green almost yellow color. Black olives are harvested after they have ripened when not only the outer skin is a dark color but also the fleshy "meat" layer between the skin and the pit but before the the olive looses its consistency and begins to shrivel.
Some varieties of olive trees can produce both good green or black olives depending on when the olives are harvested. It is more common however that any given variety produce either good black olives or good green olives. Also some olive tree varieties produce olives that are not suitable for either green or black "table" olives and are used only for the production of Olive Oil.
It is also important to know that neither green or black olives can be eaten when picked right off the tree. Both need to undergo some sort of treatment before they can be consumed safely.