To test rocks, just use a test reagent, many of the nitrate and nitrite reagents are hydrochloric or acetic acid, check the label for the words or look for a little dissolving hand. No need to buy a whole bottle of muriatic when most of us have the test kits already.
9% acetic pickling vinegar works better than 'regular' vinegar, which is usually 5%.
Make sure to scrape the rock's surface first, too, different minerals can coat rocks over time and give a false result with the fizz test. If it fizzes once, scrub all the crevices well, soak it in vinegar for a day and test it again.