Are some calcified fragments tied to Wormsinger Rune and the Skyburners Pass? I have them both and been waiting for my friends to pick up the slack and up their light.
Yes they are.
Using Wormsinger rune an event will start. Killing the named enemie should give you an uncharged worm piece. That piece charged with 50 kills in the "dregboat" will transform into a random named chest key. That first named chest you open will have the fragment you need.
The skyburners pass is a timed event chain that will give you a key for a locked chest. That one also will have the fragment you need.