One of the most interesting crystals: have deep green color, beautiful highly symmetric shape and stable on air. Unfortunately, I was not able to prepare aluminium analog: Na and K alumooxalates just crystallized separtely.
Potassium twin of ammonium zinc sulfate, visually indistinguishable from it. I suspect that precise measurement could reveal some subtle differences in angles between faces, but for the naked eye these compounds look the same. Similarly, baking soda can be used to improve crystal quality (see "Growing" section).