How To Encase An Opal
Seal the end of a small tube (9mm heavy wall works well)
Insert opal so that it is laying at the bottom
Heat the bottom of the tube until it is melted in and the opal is encased
Anneal at 1150 - 1225 for an hour
Leave it in the kiln to slowly cool over night - this is a crucial step
Don't open the kiln during the cool down period.