In a medium saucepan, combine raspberries, sugar, the juice of one lemon, and lemon zest (optional).

Mash thoroughly using a potato masher or fork.

Bring to a boil over medium-high heat, stirring constantly.

Reduce heat to medium and cook 15-20 minutes, stirring often. You will know jam is ready when it thickens and darkens in color.

Remove from heat and allow to cool 30 minutes before transfering to sterile glass jars.