We generally think of the fruit as an apple - a symbol of sexuality. However, in some regards an orange would make more sense, as the tree bestowed knowledge and an orange could be regarded as a symbol of the sun, gold, light and illumination. (This is touched upon in the Mary Magdalene: Princess of Orange book which I mentioned in my last post - I'll review it on here when I've finished reading it).
In some works of art the fruit is actually depicted as an orange as well. The image beneath is by Lucas Cranach the Elder.
Note the difference between the fruit on the Tree of Knowledge in the foreground and the fruit on the trees in the background.
I also came across this painting called The Fall by Michiel Coxcie.
No doubt there are many others out there.