You'd almost certainly need a license from each college or university. Jostens probably has such licenses; they've been around since I was in high school (and probably for a long time before that). It takes a long time to put together such a ubiquitous business, but once you've accomplished it, you've got a head start that no competitor can overcome.
That said, you're not prevented from using any particular color of gemstone, or any particular font (as long as it's not a copyrighted design specifically owned by a university). But if you make jewelry using a university's name or slogan, or jewelry that a reasonable consumer would think came from / was associated with the university, you'll probably run into trouble. Possibly even trouble with Jostens, since that's whose business you'd be interfering with.