Perhaps because is not Mond process Carbonyl Nickel, that is to say, it does to have deep light absorbing crevices, but is, as you can see from the SEM, extremely smooth and spherical. In this form, one might expect particles to act in a specular fashion. The photo is taken with a smart phone in HDR mode with light glancing off these spheres into the lense, also, the S7 has a very aggressive noise reduction algorithm which makes things like dust a bit mushy.
In future will try to take with Canon 7D with a bright lense. Will try to take more photos when the radiographic plates have been exposed.