From what I have found, I think hydropicus is derived from the word "thirsty" or "charged with water" which means that it is a stone that probably absorbs water. A meloceus is believed to be a diamond which some ancient writers believed would dissolve in a young goat's blood. The aspilates is a flame-red stone. Hopefully I have been useful to you. : )

I'm going to guess, then check. Based on some of the others and the context, I think these are names of "stones" that were thought to have magical properties. I'm putting "stones" in quotation marks. Garnet and selenite are minerals, found in rocks. The passage itself explains that "bezoars" are hard concretions, the result of biological processes, found in animal bodies. Since these are old beliefs, I'm guessing that this is an historical novel set in the past. The writer is probably using some out-of-date words. I've heard the word "bezoar" before, even though I didn't remember what it was. As for your three, first I'll check an electronic Merriam-Webster's Unabridged, and, if that doesn't work, I'll try a web search. But the main point is: these are not words a native English speaker would normally be expected to know. They might even be invented words. Webster's doesn't know hydropicus, meloceus, or aspirates (in the sense of a stone or crystal or mineral). Oh, for goodness sake, I might have known this... this is a passage from "The Picture of Dorian Gray," by Oscar Wilde. And the third one is "aspilates," not "aspirates." This web page http://gwydir.demon.co.uk/jo/minerals/dorian.htm explains about thirty of the the minerals and gems mentioned by Wilde, but says "I don't know what hydropicus, aspilates or meloceus are." He might have made them up, or researched minerals whose names are so obscure that nobody could be expected to know them.