Tanzanite

Tanzanite was first introduced to the market in 1967 by Tiffany and co.  The intent was to be an affordable, alternative to the "cashmere" colored sapphire gemstones.  Tanzanites quickly became a collectable stone, in it's own right.  Perhaps it's the mystery and rarity, given that it's a one source gem, coming only from Tanzania from a mine that only stretches 8 miles in length. 

Nowadays, Tanzanite has been heavily mined and it's very unusual to get dark blue or dark purplish blue gemstones.  Most commerical Tanzy's on the market today are medium to light purple and expert gemologist predict that it will be a one generation gemstone: meaning it's quickly being mined out.