Mix all ingredients in a glass or ceramic bowl. Your mixture should be wet, but not so wet that the salt starts to dissolve.
Spoon mixture into one mold cavity and press down with the back of a spoon. Turn out molded mixture immediately onto paper-lined tray (start in the top left corner because once they are out you can't move them until they are dry).
Repeat process until all the mixture has been used. Allow gems to dry for 48 hours, turning after 24 hours (try to avoid handling them before then as they are fragile when wet).
Store them in a plastic zip lock bag or glass jar for color and scent longevity or, if giving them as a gift, place them in a cellophane bag finished off with a pretty ribbon.
To use, drop a whole handful of bath salt gems into a bucket of water for a relaxing foot soak, or dissolve the entire batch in a warm bath.