The piece of jewellery she wore on her right hand was a giant emerald ring.
She was first spotted wearing the bling on July 25 when promoting cosmetics giant Estee Lauder’s new Dimensions Skincare line at a mall in Florida, reports aceshowbiz.com.
The 41-year-old has been flashing the speculated engagement ring in photographs she shared on photo-sharing website Instagram.
Neither Mendes nor Gosling has addressed the engagement rumours.
The actress’ representative also has not yet made any comment on the matter.
Mendes and Gosling first met in the fall of 2011 on the set of their drama film “The Place Beyond the Pines”.
Despite their headline-making romance, the couple has been acting very private about their personal life.
In September 2014, they welcomed their first child together, daughter Esmeralda Amada.