I wouldn't call it a scam. They do push the tanzanite like crazy on cruise ships, but the price is rising each year, particularly for the better quality stones. As far as rarity goes, Alexandrite is the rarest stone, and very expensive. However, I doubt anything will truly rival the popularity of the diamond, even though they really aren't rare, and their distribution is carefully controlled.
We went on a cruise with some friends several years ago, and they decided to buy some tanzanite as an investment. Problem was, they bought jewelry. The price may go up a bit, but used jewelry is much harder to sell than loose stones. They sure are pretty though!