Somewhat partial answer and thoughts:
I think the solution might be
With the following explanations:
I'm a snake on an island,
I think this refers to the sign "?", which looks like a snake, with a dot below (the island).
I'm certain letters (possibly with something to end).
Letters are characters, as is "." which is something to end.
I'm in the DNA of a tree,
For this one, I was thinking about family trees, in which case the DNA is similar, so this would correspond to the character "=". On a second thought, I think the "°" (degree) fits better, still with the idea of the family tree
And the queen of a land under the sea.
land under the sea makes me think of sand, hence I think ampersand "&" might apply, although I'm not sure about the queen part.
I'm even a stone of some kind,
You can say a stone can be "♯" (sharp).
Now tell me, what am I, before I blow your mind?
"!" is sometimes called a bang, fun fact: "‽" is called interrobang