Endocarp

From Freepedia

Endocarp is the hard inner layer of the pericarp of some fruits that contains the seed. It is especially, but not exclusively noticeable, in members of the subfamily Prunoideae including sloes, damsons, apricots, peaches, plums and cherries.

In nuts, it is the stony layer that surrounds the kernel of pecans, walnuts, almonds etc. and which is removed when eating.