As far as a rule of thumb for unknown material, all I can think of is “Assume the worst.” 🙂
Yes, there is a well-developed methodology for identifying unknown gem materials. If you have the time and money, a course in gemmology will teach you a lot. One of the most important free resources is gemologyonline.com and their forum.
Are you a member of USFG? If you are, there is a series of articles, “Kitchen Counter Rock and Gem ID”, by Sue Lichtenberger, MD, starting in the December 2015 issue of the USFG Newsletter.
If you’re not a member, it’s the best $18 you’ll ever spend.