When looking for an opal you want to make sure it has lots of color from green, pink, blue and sometimes a purple.  Some may have more of one color than the other.  I have seen some that are mostly blue which are very pretty.  If you purchase an opal ring, be careful when you wash your hands because opals can dry out and crack or become brittle.  I recommend removing the ring so the stone does not dry out.  They are also softer than other precious stones so they can break easier when hit against something.  Make sure to ask questions when bidding on any opal jewelry like the color and how well it has been taken care of.