Answer: Black spots can be caused by insects, a disease, or may even be the excrement of houseflies. Without seeing the plant it's really difficult to narrow down the cause.
Try wiping the spots off with a damp cloth. Fly specks should be easy to remove. Some fungal spots are easy to remove, as well. If the leaf surface is damaged, or you find a sunken area, the remedy becomes more difficult. Damaged leaves can be picked off the plant. If many leaves are affected you may want to cut back whole stems to stop the spread of disease. After you've removed the damaged leaves, give your plant plenty of TLC. New stems and leaves will develop if the root system is healthy.
Good luck with your jade plant!
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