Natural, unpolished turquoise cabochon dome ring set in 18k gold band. Unusual & unmarked. Maker unknown. Antique estate find and likely 100+ years old based on the home and other antiques. Because this is a dome and sits fairly high there is room to fit up to 5.5 size finger. 

Tests as 18k gold

Size 5

Dome 3/4" x 1/2"