Dried pine sap = Amber!! Yes it can dry, but a powdered additive, like charcoal or even dust hurries the process. The tiller of my Avatar bow is a piece of 125 year old Southern Yellow Pine that was cut down more than 50 years ago. It has a few sap pockets that will break teeth out of a saw blade, and the sap is diffused throughout the wood making it VERY hard to cut and work.
My first though when I saw the picture was that the technique was "kolrose" carving (like scrimshaw but on wood. However those lines are too wide and deep. Still, the technique might apply. The grooves could be 'painted' with some sort of varnish, and then finely ground charcoal blown or poured into the wet lines and the varnish allowed to dry before being wiped clear.