Cut a lemon in half and dip the flesh in salt. Rub hard on the stain.

You can remove stains from marble with a paste of baking soda, water and lemon juice. Rinse with plain water.

Stain removal from stone is a professional job I'm afraid as it would have seeped into the porous particles. The first step is to identify the stain. Is it oil, ink, paint, dirt, wine, coffee, rust? Check with a marble company and ask what they would do, if there is a product that you can buy and try or if you need to get a pro in.