Cut the pineapple as small as possible, as the pieces do not get completely soft when cooked. Cut the basil as small as possible.
Bring the pineapple to the boil and cook for another 3 - 5 minutes until the pineapple becomes softer. Stir again and again. Mash the pineapple with a potato masher or something similar.
Add the preserving sugar and bring to the boil again, simmer for 3 minutes. Remove the saucepan from the heat and stir in the basil.
Pour the jam into thoroughly boiled glasses, close and let cool.