Vintage 14K gold-filled heart shaped locket. The mark is the letter M in a diamond, possibly for "Dolly Madison", a brand used by Marathon Jewelry. Total weight is 2.5g, marked "1/20 14K".
The locket is 1/4 inch by 5/8 inch, inscribed with a flower. It's on a chain that's 18 inches.
Condition is excellent.