Using your fingers, pinch individual grapes, separating skins and pulp into 2 saucepans. Bring grape pulp to a boil over medium heat and boil, stirring frequently, until soft, about 10 minutes. Press pulp through a fine sieve and discard seeds. Set aside.
To the saucepan containing grape skins, add enough water to prevent sticking. Bring to a boil over medium heat. Reduce heat, cover and boil gently until skins have softened and liquid is nearly evaporated, about 10 minutes. Combine skins and pulp in a large, deep stainless steel saucepan.
Prepare canner (water bath). Sterilize jars and lids.
Stir sugar into grape mixture. Bring to a boil over medium heat, stirring constantly to dissolve sugar. Boil, stirring frequently, until mixture thickens. Remove from heat and test gel. If gel stage has been reached, skim off foam.
Ladle hot jam into sterilized jars, leaving 1/4-inch (0.5-cm) headspace. Wipe rim. Centre lid on jar and screw on metal band.
Place jars in canner, ensuring they are completely covered with water. Bring to a boil and process 10 minutes. Remove canner lid. Wait 5 minutes, then remove jars, cool and store.