The Bible never says what kind of fruit it was. But the Bible is not to be taken literally, in any event. The subject of the Bible is spirit and our human languages are poor methods for describing spiritual things, because they were created from physical experiences.
The authentic Kabbalists of today (Bnei Baruch) tell us that Kabbalists wrote the Bible in code called the "Language of Branches" in order to relate this difficult topic of spirit. I'm only just beginning to learn this.
My own current take on the Garden story is that the Garden of Eden was itself a spiritual place and not a physical one. The fact that God said that Adam would die on the day he ate of the forbidden fruit is one powerful clue to this. Why? Because the Bible goes on to say that Adam lived for 930 years, most of that outside of the Garden. So, we cannot take the death in the Garden as a literal, physical death.
The spiritual death in the Garden mirrors the spiritual rebirth of which Christ speaks. Nicodemus misunderstood and thought Christ was talking about being born again from the mother's womb. Not so. Spiritual death in the Garden; spiritual rebirth with our salvation. The body has little to do with it all.
So, the fruit cannot be taken as a literal, physical fruit.
"Fruit" also has the meaning of "product" or "end result" of something, like "fruit" of one's labor.
The Kabbalists talk about a Tree of Life which is also mentioned in the Garden, but their Tree of Life is merely and only a conceptual matrix of ideas like wisdom and beauty.
The evil tree was described as "knowledge of good and evil."
Knowledge is frequently used in the Bible to describe lying with (as in sex).
Wallowing or lying with the dichotomy "good-evil" is another way to describe a part of human nature we call "ego" or "self-concern."
Thus, the fruit of these dichotomous forces is ego itself.
So, the fruit Adam and Eve bit into in the Garden was the self-concern (ego) which made them spiritually blind and disconnected from God.
The only cure for ego is its opposite -- Altruistic Love (agape).