You guys, taking things too seriously
The Pandora Gem will probably flash pictures, codes, or chemical formulas (for making the immortality potions/medicine) on a wall when struck by moonlight. Hence, the "tears" of the gem.
Though I wonder what real life reference did Gosho use for the Pandora Gem. All I know is the Pandora's Box from the Greek Mythology. The Pandora Gem might symbolize the only thing left in the box, w/c is hope, or maybe one of the other evils that came out of the box (anger, despair, lust, etc.)
The reason why I hate Kid is because he lives for revenge. He even said himself that he hated what he was doing: stealing gems and stuff (in the last MK chap, if my memory serves me right), but does it for the sake of finding the people who killed his father. But he probably forgets his grudge for a while when great detectives like Conan and Hakuba challenge him, making heists more interesting for him.