What qualities must a mineral possess in order to be considered a gem?

It depends on 4 characteristics:
Beauty
Durability
Hardness
Rarity.
Beauty is the clearness (clear doesn't always mean worth more), color and brilliance, aka the brightness. Durability and hardness are quite the same, the harder and more durable, the better (think of diamonds are the hardest known natural material). Then rarity. Some minerals are very rare (big diamonds). The rarer, the pricier of course. Also the way it is cut makes a big difference.

Ch4 is nonpolar and insoluble in water. since all of the hydrogen atoms are evenly spread around the carbon atom even through the c h bond is polar all the poles cancel each other out.