Effie Calavaza, or Effie C. jewelry, is very popular and a lot of people collect it. I think she started using this hallmark around 1970 when her husband Juan Calavaza died; before that they shared a hallmark. One thing to keep in mind is that not all jewelry hallmarked Effie C. was made by Effie herself. She passed down her designs and hallmark to “authorized” family members who she allows to use them. However, at least one family member (through marriage), who is not Zuni, has been making jewelry in her signature style and stamping it “EFFIE C. ZUNI” without her authorization and in violation of the IACA.
I don’t see anything about your ring to indicate specifically that it is one of the fraudulent Effie C. pieces, but I also am not an expert on Effie C. jewelry. I do know that the genuine pieces use almost exclusively turquoise and coral, and always use rough/natural cut cabs. So unorthodox stones such as onyx, opal, and white buffalo, as well as perfect-cut or very flat cabs, are usually signs of the fakes. Your ring passes both of those criteria.
Here is a court affidavit regarding the unauthorized fakes; information about Effie’s jewelry starts on p. 12: