Spray a highlight onto the stone in the darker area. Rub the highlight until it's really not white anymore, you want a soft glowing effect. The actual stone is done, you can resize it down and use it now. If the selection isn't still loaded, load it again. Right click on the layers palette and add a new layer, drag this layer to the bottom.
Flood fill the selection with white.
You'll need to play around with the settings, the radius needs to be 25, I personally prefer a soft bevel. Use the percentage boxes and enter a number between 30 and 50.