In a heavy-bottomed, non-reactive pan, mash the grapes, a handful at a time. Pick out the skins, and put them in a small bowl. Place the pan over medium heat, and bring to a boil. Reduce heat, cover, and simmer for 15 minutes.
Press the pulp through a sieve to remove seeds. Combine the pulp with the skins. Measure the volume of the mixture – you should have about 3 cups. Return the pulp to the pan and add the sugar. Heat over medium heat, stirring, until the sugar has completely dissolved. Raise to medium-high heat, and boil, stirring often, until it turns a glossy dark purple and thickly coats a spoon.
Remove from the heat. Ladle the jam into pint or half-pint jars and seal with lids and rings. Process for 10 minutes in boiling water.