It is roundish in the sense that it looks more round than oval, but it's not totally round. It's 10.5mm in its largest area, but slightly over 10mm in other sections.
It's flattened on the bottom and would set into a ring perfectly. I believe that big it is bezel set it would make it look perfectly round.
The item "Natural 2.85 Carat Round Green Emerald Genuine Loose Gemstone 10.5mm Cabochon" is in sale since Wednesday, September 13, 2017. This item is in the category "Jewelry & Watches\Loose Diamonds & Gemstones\Loose Gemstones\Emerald\Natural Emeralds".The seller is "ceylontradingcompany" and is located in Norwich, Connecticut. This item can be shipped worldwide.