User blog comment:Wildoneshelper/Guess the Object (Round 2)!!/@comment-17897872-20140613070205/@comment-4189499-20140613122231

Technically, a substance with a pH over 7 is called a base. An alkali is a special kind of base which can dissolve in water and has an OH group as part of its structure. Another definition is a "proton receiver", but that's a bit too complicated for our purposes here.

An ionic substance is something made of positive and negative ions, ions being atoms (or molecules) with a charge, as they have either gained or lost electrons. Ionic substances are usually made of a metal and a non-metal, for example table salt, i.e. sodium chloride, which is made of the ion of the metal sodium and the ion of the non-metal chlorine.

Diamond is not alkaline. It is neutral. The science-y reason why this is is because the carbon atoms are all covalently bonded to each other (basically, held so tightly that they share each others atoms as if they were their own) and so cannot come loose and disrupt the balance of H+ to OH- ions, that balance being what pH is.

It can't be sodium carbonate, because Wildoneshelper has already said the thing doesn't have sodium ions, but it is a good guess. Carbonate ions do neutralise acids, making them by definition basic (or alkali), while sodium ions do nothing once dissolved in water, so I think it would be considered a basic salt. Hmm, could it be potassium carbonate? Potassium, like sodium, does bugger all, while it would still have the neutralising carbonate ion there to make the salt basic. I think.