The crystals can be attached to the bottlecaps when you are using one of the resins like ET Lite or Rio doming resin etc. You drop them when the resin is just starting to set so they don't sink to the bottom. When using the resin drops on the bottlecaps, you won't be able to apply the crystal
Yes! they are sold at Michaels and for this purpose you should look for the one that are NOT hot fix!
Hope this helps!