Without a pic, there is no way to identify it....positively, that is.
See if you can get a good shot of it, then come back here.

"Cocoons" are generally fuzzy and/or covered in silk strands. If you are saying yours are hard to the touch, they might be some other kind of pupae. In the absence of photos for us to work with, please describe your mystery item in detail, size, color, etc. Also what plants are you finding them on, and do they show damage that would be consistent with some kind of caterpillar infestation?

One site that may help you is BugGuide.net. I am linking to a page of pupae. You can try to find one that matches yours.

Here is a link that might be useful: BugGuide.net Pupa Search