"Here's a poultice formula for coffee:
Make a solution of 20-30% peroxide (available at beauty supple places...wear gloves Mine is called Salon Care 30Extra Lift Volume Creme. Bought it at Sally's Beauty Supply Store.) and a few drops of ammonia. Then mix in some sort of WHITE "material;" e.g., paper towel, napkin, tissue. Make only enough to cover the stain. It should be paste-like (consistency of peanut butter).
Wet the stained area with distilled water. Pre-wetting fills the pores of the stone with water isolating the stain and accelerating the removal by the chemical.
Apply the poultice to the stain being careful not to spill any on the non stained areas. Apply approximately 1/4-inch thick over-lapping the stain area by about one inch.
Cover the poultice with plastic (food wrap works great). Tape the plastic down to seal the edges. It also helps to poke several small holes in the plastic so that the powder will dry out. Failure to do this may result in the poultice staying wet.
Allow the poultice to dry thoroughly. This is a very important step. The drying of the poultice is what pulls the stain from the stone into the poultice material. If the poultice is not allowed to dry, the stain may not be removed.
Drying usually takes from 24 to 48 hours.
Remove the poultice from the stain. Rinse with distilled water and buff dry with a soft cloth. If the stain is not removed, apply the poultice again. It may take up to five applications for difficult stains.

Here's some additional tips!
For the "white stuff" you are going to use for your poultice powder base ... get some diatomaceous earth ("DE"). You can get this really CHEAP at a pool supply store or free if you know someone with a pool that uses it. It is used in some pool filtering equipment. I went to the pool supply store and they gave me some since all I wanted was a small amount.
Good info on stain removal:
From www.stone-panels.com/details/stains.doc
Iron (rust) - Poultice with Oxalic Acid + Powder + Water. May also try a product called Iron-Out (available at hardware stores). Both mixtures may etch polished marble, so re-polishing will be necessary.
Ink - Poultice with Mineral Spirits or Methylene Chloride +Powder.
Oil - Poultice with Ammonia+ Powder Methylene Chloride can also be used on tough oil stains.
Coffee, Tea & Food - Poultice with 20 percent Hydrogen Peroxide + Powder.
Copper - Poultice with Ammonium Chloride + Powder
Paint (water-based) - poultice with a commercial paint remover + Powder
Paint (oil) - Poultice with Mineral Spirits + Powder. Deep stains may require Methylene Chloride.
HTH
MaryT"

Hope that helps!! Good Luck!!

I have now added this to my clippings, just in case, although I will have my counters sealed properly when they're installed. Right now, I'm arguing with fabricators who think I'm nuts for wanting honed marble in my kitchen. YOURS are beautiful, by the way!

Today I went to wipe down our (Calacatta Extra) countertop and, like an idiot, didn't look at the sponge before wiping. Stuck to the bottom of it was a rusted Brillo pad, and it proceeded to stain the marble with a big yellow splotch. I was in shock!

I immediately checked here and found this post: within forty-five minutes of the stain occurring, it was gone! My sweet husband and three year-old went off to Lowe's to fetch Iron Out and it worked instantly.

Thank you!!!