The Granny Smith, also known as a green apple or sour apple, is an apple cultivar that originated in Australia in 1868. The fruit is hard, firm, and with a light green skin and crisp, juicy flesh. The flavor is tart and acidic. It remains firm when baked, making it a popular cooking apple used in pies, where it can be sweetened. The apple goes from being completely green to turning yellow when overripe.

Source: Wikipedia