The mystery fruit is guava. Five people guessed right and Mika of the Green Jackfruit is the randomly-chosen winner of a big $5 Barnes & Noble gift card.
Congratulations, Mika, and thanks to everyone else who played.
Mika, email me your address and I will send you the card.
To answer the question, why is it brown on the outside, I don't know. Leaf scarring? Rattling around in my produce bag? I'm not sure.
The guavas we usually get here are actually feijoas, Feijoa sellowiana, also called the pineapple guava. But the one shown in the photo is a real guava, Psidium guajava. I have heard that they can grow ok in So. California but are better in warmer areas.
I have to admit the one I tasted wasn't quite ripe and the little seeds were hard as rocks. How do guava eaters get around those seeds?