ToME: the Tales of Maj'Eyal

I couldn't combine it with a plain (white) ash longbow but I created a cristalline shield out of a plain (white) dwarven-steel shield.

Why is that? Its item description says combine it with a weapon....

R.

I think the game checks if an item has a damage value for determining whether it is a weapon or not. Bows don´t have a damage value, technically the ammunition is the weapon. Shields have a damage value for bashing (and Stone Wardens can put one in their weapon slot).

Gloves also have a damage value, so perhaps that gem works on them as well.

Basically you need to add it to a melee weapon. I guess it is not clear when it says weapon