Try checking your bag again, also try checking if any Pokemon is holding it agagin, it should turn up because you can't toss or sell Mega Stones.
If it is gone, the only way to get back your Diancite (as of now) is by trading. You need someone to give you their Diancite. However, this is unlikely to happen because not many people will want to give away a Mega Stone.
"There are some people who can clone items, imo you should try asking one of them for a Diancite."
Yes, as Qwerty said, you could also ask someone with a Diancite to clone theirs for you.
Hope this helps!