Rinse the raspberries and pat dry. Pour the raspberries and the jam sugar in a pot. Stir and let sit for about 2 hours. Puree the raspberries. Add the lemon juice and heat, stirring contantly, for about 5 minutes.
Pour the jam into jars rinsed in hot water.
Cover the jars and turn upside down. Let sit for 15 minutes. Turn right side up and leave to cool before serving.