Cartier 18k White Gold Black Onyx Square Top Tank Ring Size EU 53-US 6.5 w/Cert. This ring is from the tank collection by Cartier it is crafted in solid 18k white gold with a fine polished finish and boast a square shaped top with a high polished black onyx gemstone set in the top center of the ring. Slim shanks and wide shoulders. Material: Solid 18k white gold. Hallmark: Cartier 750 1998 I 90880 Measurement: : Top Size: 16mm across x 12mm wide x 7mm high Onyx: 11mm x 11mm Weight: 22.5 grams Stock#: 29520