I'm a graduate student in engineering, working primarily with organic chemicals. I've had coursework in inorganic chemistry, though my knowledge in this field isn't great. I was waiting for some samples to be ready for analysis, so I had the idea to make some aqua regia and dissolve a few coins that I had in my pocket. Note that all steps were performed in a fume hood rated for small explosions, in secondary containment vessels, with all the required PPE. A penny dissolved rather quickly, then I moved onto a quarter (so much for laundry) and a dime. I had some fun dissolving some other coins part way until they were left with a nice copper sheen on both sides. I also dissolved several more pennies in roughly 40 mL nitric acid, because I planned to create a nice layered suspension later after precipitating the metal. After awhile, the pennies stopped dissolving and I transferred both beakers (one with just pennies and the other with the mix of coins) into two separate, and larger 500 mL beakers. I added water, and then some KOH tablets to precipitate the metals. pH was around 13. The solution containing pennies turned out beautifully, and I centrifuged it after taking some photos to separate the solids from the liquids as my lab requires. HOWEVER, the other beaker turned jet black when I added KOH. Almost instantaneously. It was a very violent reaction, shooting a black tarry substance into the secondary container. After that settled, I formed what I believe to be ammonia crystals on top because of the odor. I vaguely remember something in class about going past the pK2, I believe and forming ammonia though I'm sure someone can explain this much better. However, I wasn't sure about the black inky substance. Could this be some copper or nickel oxide, perhaps?