I am center opal is approximately 3.54 mm and smaller opals are about 2 1/2 mm. Very nice antique 14 karat so boring from a local estate pictures do not do justice to these opals. Very nice ring. Approximate ring size is 10 3/4 but appears that it would be an easy ring to size up or down by your local jeweler. Rolling Pin Holder, Estate Ring, Vintage Silverplate, Estate Rings, Leather Sleeve, Size 10 Rings, Multi Stone Ring, Natural Opal, Gold Rings