This art technique is called scratch-art. And here is how I prepared the base for scratching:
I took a sheet of plastic transparent foil, and glued some diamond-shape pieces of colored transparent foil to one side of it. Then, I turned the foil and painted the whole surface with black tempera mixed with some egg-yolk. The egg-yolk prevents the tempera from cracking.
A base prepared this way represented the mine with hidden diamonds. And then, the little hands came to digg (scratch) in search for diamonds.
The kids had so much fun scratching the surface in search for diamonds...
And since the scratched surface was transparent we hung it on the window as a stained glass, and the light shined through the colored transparent foil as they were real diamonds!