Place the berries in a coriander wash and then drain them.
Put the blackberries in a large saucepan and mash them using a masher until they resemble a soup with bits texture.
Pour in caster sugar and lemon juice and mix together.
Place on the stove and heat the mixture until the sugar has dissolved. Continue to stir the mixture as it boils to the set point. To check for the set point, place a little bit of jam bit on a cold plate and leave it in the fridge for 2 to 3 minutes. When the jam is firm and no longer runny, it is ready.
Remove it from the stove and then pour it into sterilised jars. Let it cool down and seal the jars. Store in the fridge or in the freezer.