Board Thread:Theories and Speculation/@comment-30168130-20170115054258/@comment-4912646-20170116035037

It doesn't seem to me like Madara implanted actual rods in Obito's artificial half so much as his will itself, which allows him to grow rods out of it need be. At least, that's what I took away from that scene where Obito's right half turned black. Though we've also seen Obito deliberately generate rods from his right hand to use as weapons.

So slightly outlandish theories:
 * "Will Materialisation" is a Yin—Yang Release technique. It can produce black rods as well as living beings like Black Zetsu, meaning they are made out of the same material in different forms.
 * The reason Madara and Obito could create rods without the Rinnegan is because they both had Hashirama's cells, or the power of Uchiha and Senju in one. Obito's explanation of Izanagi to Konan seems to imply that Senju + Uchiha power is a requirement for using Yin—Yang Release, though arguably this was retconned when Izanami was introduced. This also assumes that just having the Rinnegan can grant Yin—Yang Release in Nagato's case, which is implied by Jiraiya's "six natures" line from the original printing of the manga.
 * We know Truth-Seeking Balls contain the five natures in addition to Yin/Yang Release. So maybe if you add the other natures to the black material, it transforms into the TSB, though only those with Six Paths Senjutsu or the chakra fruit's power can do so. This could be why Obito was able to turn part of Black Zetsu into an orb after stealing Six Paths Senjutsu chakra from Madara, but it doesn't explain why they are a finite resource though.