Not sure what you mean.

To unfavorite anything in your Favorites Menu you just click on it's start 

To unfavorite legendary runes, go to your rune inventory and locate the legendary runes you want to remove from favorites. Then, click on the favorite button or star icon associated with each rune to deselect it as a favorite. Follow My Health Sharp 

This was before they fixed the bug in game.

where do you go an how do you get precious dust

You have to crush legendary gems.

Someone will have to tell you where, and how many you need, unless you can find it  in the game somewhere,  ie where it is done, or in the forum data base .

I just  checked myself, as I thought  I had recently come across it.

Workshop, and it was 2 for Topaz, which appears first in my list.

https://vikingsweb-support.plarium.com/web/en/guide/items/items-for-the-clan-stronghold