Are no longer showing up in reagents for my scrap of the soul cloak. Haven't been playing for a couple/few months and logged in today, used the Nth metal from a few BB'S to go from 99 to 100 then went to keep leveling my artifact with the stones but they're not showing up.
Did you have a successful breakthrough at 100? If an artifact is waiting for a breakthrough you can't feed it any more xp. Otherwise it might have something to do with them being protected, like Lorax said. Similar to how you can't use a seal if it's marked important.
Mine was same after restart yesterday.. could use normal Nth Metal but not Sin Stones.. will check again