Try soaking it in pure acetone. If that doesn't do it, you might try (gently) picking it out with a rose thorn. Chris
If you have never used it before, use with great caution in a well ventilated area. It's also good for getting crud off copper coins. After a soak in the acetone, follow by a rinse of distilled water. Or just boil your own water so it's contaminent free.