Quite a few higher-level spells require x GP of gemstone or gemstone dust in order to cast — I will not bother listing them here, as it'd just take up space in the question.
However, the value of a gem is primarily determined by Cut, Color, and Clarity (or lack of imperfections), not its size. Considering that the same base material can vary widely in these regards from industrial-grade stones that are practically worthless to a jeweler, but still useful for their material properties up to the finest gemstones money can buy, what minimum standards are required of a gem in order to be usable as a spellcasting material component?
Must it be cut before it can be used, or can uncut (raw) gems be used for casting, straight out of the ground? Could a wizard cast a spell that say called for a sapphire or a ruby with a crystalline lump of industrial corundum (synthetic sapphire) instead of a natural gem? Would a large shard of bort be useful for spellcasting in lieu of a gem-grade diamond?
Citations strongly preferred here, by the way — if you need to dig back into the archives, that's OK, but right now, this is something that I would not even know how to consider from a DM or player perspective.