The attached image is 100,000 times magnification of a chrysotype, taken with a scanning electron micrograph, provided courtesy of Dr. Malik at the Institut fur Nanotechnologie in Karlsruhe, Germany.
What you're looking at are celulose fibrils with nanoparticle gold (the white bits...) embeded - this is what makes chrysotypes, and by extension, platinum prints (and several of the others mentioned above) so permanent. The image is embeded in the base material - it is not, as in the case of silver gelatin, attached in a layer on the top of the paper (which will eventually detach).
The thing that makes platinum and gold stand out for me is the stable nature of these noble metals - they are two of the most stable elements available to us.