Just think of what a whetstone is, dude. Anyway, it's for swords, of course.
Dude there is so much to unpack here. First, why couldn't you use it to sharpen a dagger? A stone that can be dragged along a blade magically refused to work on a smaller blade, is that your argument? (Spoiler alert: people absolutely do sharpen knives with whetstones genius.)
Second, I didn't say I couldn't use it on a dagger, I said I was able to attach a gem to a dagger.
Third, you didn't even say how to use it lmao...
I have tried going to weapons from in my inventory, like the way you would put armor plates in armor, but I see no option to sharpen ANY of my weapons (and yes I have swords.)
I also tried to go to the whetstone and "use" it, to see if it would maybe ask what you want to sharpen with it.
I also tried interacting with the anvil and my workbench to see if either of them have some kind of relevant option to utilize the whetstone, but no dice.