Vintage Brooch with Rhinestone Drop

This pin is a recent find at an estate sale. There were many pretty pieces but this one caught my eye. It's based on nothing but intuition, but I always feel that this type of construction and material indicates age (earlier than mid-century). I would really appreciate any information. Maybe a web site that identifies construction techniques of the different costume jewelry eras? I never know what to look for when shopping estate sales. Sometimes I buy to resell (to support my habit) but mainly I just buy stuff 'cause it's pretty!