If you want to isolate parts of the texture, you need masks as Martin says. Not sure why you want to use the knife tool. If you have a photo editing application like Photoshop or one of the free open source ones, you can build masks for the parts you want isolated and then integrate them into your materials and nodes. That is usually how we get finer control of our material. In your case you make mask for the gold and text parts. Remember white means influence and black means no influence.
