5. Duplicate the layer (Ctrl-J) or (“Layer” -> “Duplicate Layer”)
6. The duplicate layer should now be selected.
Go to “Image” -> “Adjustments” -> “Levels” and use this settings.
7. Set the blending mode of the duplicate layer to “Overlay”
8. Create a new layer (Ctrl-Shift-N) or (Layer->New Layer) and fill it with the color you want your marble to be. I used #E1DFF2.
Set the blending mode of this layer to “Linear Burn”
9. Finally select the Bottom layer and change its opacity to 10%.
This will give your marble a slight color variation making it look more realistic.
10. Merge all the layers and you are all done.
Here is my final result