I think someone of Tolkien's age, raised when horses were omnipresent, would readily think of this....
Also, taking this another degree further, picking up on Verlyn Flieger's "Splintered Light", is Saruman also hinting that he wants to tame light itself!! Is he harking back to Fëanor? I sense Tolkien surely is.... given the multifarious light imagery throughout his Legendarium in general, and LotR in particular... Now, maybe the Palantiri were not yet on Tolkien's mind when he originally wrote this, but they were during later revisions... and the Palantiri, like the Arkenstone of the genus Magic Jewels.... which are means of refracting (breaking) light... whether original to them or borrowed