What am I trying to mine, but can't, in Pre-Hardmode? [duplicate]

Solution 1:

If by shiny you mean glowing, it's either the demonite or crimtane ores. You will need at least a gold/platinum pickaxe to mine it, or you can also use bombs/dynamite (make them sticky with gel for more precise mining).

Outside of a handful of patches found in the corruption/crimson, it is so rarely found underground that you probably won't be able to craft anything meaningful with it. The best source is actually from the first couple of bosses you will fight. You will also need an ingredient only found by slaying a boss to combine with the ores to make the armor or pickaxe.