User blog comment:Wildoneshelper/Guess the Object (Round 2)!!/@comment-4189499-20140613123540

Random stab in the dark here: calcium carbonate? It's common, and it is a basic salt (I just checked), but it's not technically alkali, because it doesn't have an OH- group. Calcium carbonate is also called chalk, although it is no longer found in classrooms as classroom "chalk" is now mainly made of gypsum instead, according to both QI and Wikipedia. It is however found naturally as limestone, and as chalk, which fits a couple of other categories. Technically, it is a crystalline structure when solid, as it is an ionic substance, but I am assuming that question may again be due to the common definition of a crystal, not the chemical one. It's white, satisfying that need, but I don't know how it would relate to biology. But then again, I'm no biologist and don't really care about biology anyway. So, yeah. It fits a lot of those criteria, though is in grey areas for a few.