I didn't know these were a thing, but have learned that they very much are, some quite finely done, this one more humble but I think pretty charming still.  Essex crystals are also known as 'reverse intaglios, ' made of a cabochon of colourless rock quartz that has been carved on the flat side after which the details of the design are painted, creating a three dimensional 'trompe l'oeil effect'. I believe that is the case here, or at least it was made, early int he 20th century I would guess, to look that way! And with a nice big red tongue! 

2" l x 9/16" w. Shows age and wear, with some rusting to the metal, but still looks great to me and completely wearable still.