The stone loses its power to some extent.
This is a fun test that everyone can try. All you need is an old umbrella.

https://www.youtube.com/watch?v=PXQEAXcXL1M

This is a whole lot better than having nothing, In my opinion.

As Ratman says, the umbrella would be great against glancing slingstones.

This is probably why there are depictions of slingers using hide or textile cloaks/garments wrapped around the forearm, likely to protect themselves against glancing slingstones, arrows and javelins. Against direct impacts though, not so much.