Place the grapes, sugar, and lemon juice in a heavy-bottomed saucepan. Start on low heat to slowly dissolve the sugar, stirring frequently. Gently crush some of the grapes with a wooden spoon or a potato masher to release their juices.

Once the sugar is fully dissolved, use an immersion blender to blend the mixture, leaving some chunks if desired. Increase the heat slightly to bring the mixture to a gentle boil.

Continue cooking for 30 to 45 minutes, stirring often, until the jam thickens. Test the jam by dropping a small spoonful onto a chilled plate; if it wrinkles when cooled, the jam is set.

Remove the pan from heat and let the jam cool for a few minutes. Carefully ladle the warm jam into sterilized jars, leaving about 1/4 inch headspace.

Seal the jars immediately while still warm. For longer preservation, process the jars in a boiling water bath per standard canning procedures.