[Turbo_pro]

Heart shaped diamonds, although symbolic of a deep felt expression of love, tend to be a hard sell. A heart shaped solitaire lacks imagination.
By the look of it I would guess 7 or 8 mm across making it close to a carat and a half.
When I as a jeweler (in another life) I would generally suggest that resetting a heart shaped stone would increase the resale value. Perhaps a pendent.
Mount it a heavy white gold triangle setting (much like one might use to set a trillion) and dangle it from a white gold 3.5 mm wire or omega.
Who knows, you might like it well enough to wear it. If the guy was a tool you earned it.