This 11 by 8 inch oval serving platter is in the Depression era Moderntone pattern by Hazel Atlas, first produced in 1934 and characterized by stepped concentric rings. The blue color is called Ritz Blue. There is a tiny, shallow fleabite under the rim and a scratch on the underside at the center (this may be a straw mark rather than a scratch). The glass is still glossy.
Hazel Atlas Cobalt Blue Moderntone Oval Serving Platter