You can date these by the pinbacks. Victorian era jewelry didn't have the secure, latching clasp that we have today. What you want to see on a piece of Victorian era jewelry is a "C-clasp" like this:
Sometimes a C-clasp is more in the shape of a tube like this:
The problem is that they can pop loose and fall off without you noticing when you are wearing them. I usually pin the brooch in place and then put a small piece of duct tape over the C-clasp. That keeps the pin from popping out.
