As I mentioned previously I am having diamonds go black on me when
I am rhodium plating. They are fine during working and then
electro cleaning but going black in the plating.
As Leonid asked, are you sure they’re diamonds? But I’d also wonder
whether you’ve encountered coatings of some sort, or an acid
sensative fracture filling. There’s no reason I know of why a normal
diamond would react to rhodium plating solution, so something else
must be at play. Most likely, what’s turning black isn’t normal
diamond. The only other thought is that a very small percentage of
diamonds, those with boron as an impurity (which makes them blue or
grey) are semiconductors. That makes me wonder whether such a stone
might be able to attract a light rhodium electroplate. But the much
higher electrical conductivity of the metal itself would make that
unlikely, I’d think.