Gemstone Classification

What makes a gemstone precious or semi-precious?  Mostly the classification between the two are for convience and in the past they were loosely based on the hardness of the stone although this system has many errors.  Now classification is more determined by fashion and rarity.  This is also inconsistent as rarity can also fluctuate with the finding of new mines or deposits.  Opal by definition of rarity could be labeled as precious even though by most it is considered semi-precious.  Other than the obvious diamond, rubies, emeralds and sapphires are considered precious.  Rubies are of the most rare and hold world wide acclaim.  Curiously though emeralds are priced about three times higher due to the naturally occurring chemical color-changing factors.  

Semi-precious stones are considered everything else that someone might deem to be pretty.  The precious stones seem to be a pretty elite group.