WordReference Random House Unabridged Dictionary of American English © 2020
hard mass, 
  1. Jewelrya hard glass used for imitating gemstones.
  2. Jewelry(not in technical use)
    • any glass for imitating gemstones, esp. emerald.
    • any of certain synthetic gemstones.
Also,  hard masse. 
'hard mass' also found in these entries:

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