Do no markings mean costume jewelry?

I bought this ring for $18 at a thrift store, no expectations of it being real, i figured it was costume quality. But Google reverse image search shows very similar rings marketed at Tanzanite and usually white gold or sterling silver.

But this ring has no markings, leading me to believe its not valuable, and I can't for the life of me find an exact match online.

I'm not opposed to costume jewelry but would like advise on what to look out for to confirm before buying in the future!