Update from Ngreth:
"Yes. The Crestra's Empowered Elegant Earring combine is intentionally failable. And if you Fail you get the Crestra's Elegant Earring back. The failure chance is LOW if you have a 300 skill but non-zero. (Around 3% (not exact) fail chance at 300 skill)
Other Raid FINAL combines are no fail.
Other raid SUB combines are not no-fail AND high skill. They also sometimes return the diamond. Sometimes they only return the crushed diamond dust, but this is on the "alt fail" which is when you miss the fail the combine but only by 5."