Sorry for another post, I should have posted this one from JanTink - it is my favorite way of doing polished stone:
There are lots of different ways of doing it. I'll tell you mine, in which I combined the best features of different people's methods to come up with my own, which I honestly thinks looks better than most of the polished stone I've seen.
You need:
Cosmetic sponge wedge
Clothespin (optional...use it to hold the wedge; keeps your fingers clean)
Rubbing alchohol (works better than isopropyl)
Two or three colors of reinkers
One type of metallic reinker
Glossy white cardstock
Get your cosmetic sponge wet, but not soppin' wet with rubbing alchohol. It should feel moist...somewhere between damp and dripping.
Put a couple of drops of reinker in each corner of the cardstock. Put some of each color in each corner. Pounce your sponge up and down in the colors, spreading it over the cardstock, leaving some areas white to light, and making other areas darker. It should look something like marble.
Once that is ready, put some of the metallic reinker right on the sponge. It should be a VERY small amount...just a dot. Now pounce this up and down on the cardstock, working to disperse the initial dot of reinker you will get the first few times. What should happen at this point is that the metallic will "spider" into the rest of the color on the cardstock. Don't overdo this effect. Simply go over the entire cardstock once and STOP if it looks ready.
Once it's dry, spray with a clear acrylic spray like Krylon Low Odor Matte Finish.
The difference between my method and others is that most people put the colored reinkers on their cotton ball or whatever and it ends up looking like a lot of little dots all over their cardstock. This way it spreads more evenly and if you have dots of color in the corners where you put the reinker, why you can just trim them off! Cotton balls I don't like because they leave shreds of fiber, plus they just can't hold the same amount of alchohol as a sponge wedge!
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JanTink
http://jantink.stampinup.net