JOhn,
What you have are probably opal triplets, not black opal. They appear black because they have a black backing. A triplet is a "sandwich" of a clear top stone, usually quartz, then a thin layer of precious opal, and then a black backing. They are glued together. In your case, the glue has broken down. A good lapidariast can re-glue then and fix them. If you live in the Southern California area, I can reccommend someone.
As far as value, triplets are worth quite a bit less than solid opal. They can range from a few dollars to a few hundred dollars, according to their quality. To store them, store them dry and never in a safe deposit box. Good glue should last a while. To sell them, that is always challenging. Check out ebay.com for an idea of the low end of the prices.
Jim
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