If the hinge is modern (and original), dating is somewhere between 1890s and now.
If you see a "safety pin" type clasp, the jewel could conceivably be as old as its invention (mid-19th c., but general use of this clasp in jewelry wasn't made until the 1880s, continuing in the 1890s as the form of a small extra pin on a chain).
It's crucial to determine whether clasps, hinges, wires and so forth are original. It's pretty hard to disguise a change of clasp or hinge on a brooch, and you can generally tell a pinstem has been shortened if it's slightly ragged, blunt or bent at the "pointy" end.
Working from oldest to newest, the open C clasp has been around for a lonnnnng time.
However, it's still used occasionally (these days usually on inexpensive jewelry).
HINGES If you're looking at a hinge that's familiar from all your modern jewelry and is original, that doesn't mean a lot, actually.
Hinges of this type have been in use since the 1890s, so you'll have to rely on other clues to get a more precise dating than 1890s to now.
If you haven't done much of this before, it's a good idea to examine several jewels that have already been evaluated for date and quality by someone whose judgment you trust.