Then that's not the Inside icon.
If you use format|cells|borders tab, look to the lower left corner to see those
diagonals.
Ps. If you do this kind of stuff a lot, you may want to create a new toolbar.
tools|customize|Toolbars tab
Click the New icon and give it a nice name
then go to the commands tab
format category
scroll down the commands window and drag every border icon you can see to that
newly created toolbar.
It might make it easier.