Wash the loquats thoroughly and cut off the blossom end, then cut the fruit in half or quarters. Discard the seeds and the sack that they're enclosed in. You'll need 8 cups of prepared loquats.
Add prepared loquats to a 6-quart Dutch oven. Cover with sugar, and allow to rest for approximately 30 minutes. The resting time allows the loquats to release their juices and liquifies the sugar.
Stir the lemon juice and cardamom in with the loquats. Slowly bring to a boil, then simmer over low to medium flame, stirring frequently for approximately 35-40 minutes. The color of the jam will change from yellow to dark amber. The consistency of the jam will get thicker.
Use a hand-held immersion blender to blend the jam into the consistency that you desire. However, allow the jam to cool for approximately 30 minutes before doing so to avoid being splattered by the hot liquid. Or leave it chunky, which is how I prefer it.
Ladle thickened jam into sterilized jam jars, leaving ¼" headspace. Seal lids tightly, then process in boiling water for ten minutes.
Notes
The white membrane that encapsulates the seeds does not need to be stripped from the fruit before making the jam.
If you prefer a smoother texture, remove the loquat skin before using them in the jam.
Loquat seeds make your fingers and finger nails black. You may want to wear gloves when handling the loquats.