Wash the grapes, cut them in half and put them in a saucepan with the rock candy and water and simmer for about 5 minutes with the lid closed, until a puree is formed.
Put everything in a tightly closable vessel, pour in the grappa and let it steep for about 6 weeks in a dark, cool place. Shake it occasionally.
After the ripening period, filter the liqueur and fill it in bottles.