18KT Gold Filled Smooth Heart Locket.

19.5x20.5MM or .76x.80inches. 

Brazilian 18KT Gold Filled charms are two steps down from solid gold with 3 percent real gold fused on to a brass core.  These charms have 60 times more gold than overlay or plated items.

If matching gold colors matters to you, be sure to check the description of the item you are buying to see if it's the same color.  Gold filled 14KT and 18KT are different shades of gold, with 14KT being brighter and 18KT being darker.