...I made a test. I got myself the salt crystals we once had grow in a jar (for experimentation reasons), and also got myself a bit of ordinary sugar. I first photographed the salt crystals with my Sigma lens at 300mm in the macro position: 1. and 2. Then I photographed the salt crystals with the reversed kit lens on, at about 24mm 3. And then I went about photographing the sugar crystals in the reversed lens mode at about 20mm 4. They are less cubic, more diamond-shaped To show how much bigger than the sugar the salt crystals are, here some sugar next to one salt crystal photographed with the lens reversed at 24mm 5. With these photos in mind, I now begin to believe that it is sugar in Neptune000's_thread .