^ It is called a mango stone or pit...it's basically the endocarp which is the protective hard casing that surrounds the actual seed. So the hard casing isn't the actual seed, the seed is within the stone/pit :)
It's just like with peach and apricot pits...when you open up the hard casing you can see the seeds within them.
Whoever came up with this term stone is a dork! :)
Just say seed shell. Why disrespect this great 'king of all fruits'?