Opals - Real or Imitation?

My friends love to pick up shiny things that they find they think are a good deal.  In this case, my Mom bought me small bag of what appear to be opals at the swap meet.  I'm not very familiar with how to tell a real opal from an imitation- so I'm posting them here so someone can give an opinion.  One thing I am pretty sure of - they don't appear to be doublets since there is no seam on the edge.  They are also not very big - ranging in size from about 3 mm to maybe 9 mm.

For the most part, they are pretty fiery - as you can see in the picture.  The fire does range in color with a lot of green and pink - but also some blues.  There are also a few that seem dull - like the finish has worn away somehow.

Some of the stones were obviously part of something - since you can see glue lines on the backs of some of the stones.  That is the one indicator that makes me think they may not be real -  like they were part of a costume maybe?

So, does anyone have any tips on how to tell if they are real?  I'm willing to sacrifice one if I have to (only the ugly dull one though!)  I've got my fingers crossed that they are - they are so darn pretty!