In The Inheritance Cycle, what is Tinkledeath, Angela's sword, made out of? Eragon says it may be diamond, but it never actually says what it is.
It is made out of solid diamond.
Quoting from Christopher Paolini on his website about a Make-a-wish he participated in:
The young man, Brenden, wanted a replica of Angela’s sword, Albitr (aka Tinkledeath), from the Inheritance Cycle. This posed certain problems above-and-beyond the normal difficulties of crafting a sword, because—as you may remember—Tinkledeath is made out of solid diamond. Yikes! Tinkledeath: A Make-A-Wish Story
And from the video of him speaking about this:
He wanted a replica of Angela's sword, Tinkledeath, also known as Albitr. And if you've read the Inheritance Cycle you may remember that Albitr/Tinkledeath is made completely from diamond—tip to pommel. Christopher Paolini Showcases a Replica of Tinkledeat