Starbucks Cobalt Blue Siren Mermaid Mug, often sought after as a COLLECTIBLE ITEM.. It features a distinctive embossed mermaid design in a vibrant cobalt blue glaze. Many versions of this mug come with a matching ceramic lid that doubles as a coaster. It is considered a HIGHLY COLLECTIBLE or RARE ITEM, particularly those from the 1990s. The mug typically has a capacity of around 16 ounces. royal blue ceramic mug with raised decorative pattern. Comes with a matching embossed square coaster. Stylish design for everyday drinks.