For an actual rock (which are classified differently than gemstones) http://en.wikipedia.org/wiki/Tourmaline

and her reasoning, "Actually gains value, good ones are not common (like diamonds are), awesome variety of colors"

Also said she likes aquamarine for actual gemstones (it's beryl, same thing as emeralds), but it's not as hard as diamond and wouldn't stand up to the daily abuse as well.

She also said she'd enjoy an uncut diamond mounted in a ring, but that's probably purely the geologist in her speaking.