Bill argues that one could find faith in this story because if Joseph was making everything up and not using his particular magical stone to see the characters, then he would have likely just kept making stuff up. On the other hand, if the one and only true seer stone actually worked as he claimed, then the counterfeit would obviously fail and he would be unable to translate. Lo and behold, that is exactly what happened. Yay, Faith!
Of course that all presupposes that Joseph did not know that the stone had been replaced. Bill notes that while lowing your head into the hat, there is ample time and light to see the rock in the bottom of the hat. The seer stone, being a rock that Joseph had carried around for years and had spent much time looking at and into, would be very well known to him. Every curve, every band of color would be familiar to JS. Given that, it would not be hard for JS to recognize the change, and being the smart swindler that he is, play along with Martin's little game and give himself an extra boost of credibility.
However, I don't think we even need to go that far. Most members today would classify folk-magic along the same lines as black-magic. We tend to lump it in with palm readers, tarot card readers, and whatever people who use crystal balls are called. No TBM that I've spoken with really believes that any of those things work today. In fact, most would call them scams or possibly fraud. So how do we justify an exception for JS and his special rock?
Many faithful members refer to the stone as a revelation trainer of sorts, given to help young Joseph build faith in his ability to commune with the divine. That is, JS's faith in himself and his ability to receive direction or revelation from beyond the veil was first founded in, and strengthened through, his use of the seer stone. Over time JS outgrew the need for the rock and developed the ability to receive revelation directly into his own mind.
In Rough Stone Rolling (pg 54) Dr. Bushman says
In his Master's thesis A Pathway to Prophethood: Joseph Smith Junior as Rodsman, Village Seer, and Judeo-Christian Prophet, Mark Ashurst-McGee (cited in the GT Essay on Book of Mormon Translation) saysMagic had served its purpose in his life. In a sense, it was a preparatory gospel.
Side note: McGee cites his belief that JS originally used the U&T crystals (or whatever they were) for translation. But that since the spectacles were far too large, he took the crystals out of their frames and put them into his hat. Later, for... reasons... he switched to the seer stone we know and love today. Oddly enough, he refers several times to the brown seer stone as the "less powerful" device, but never explains why one stone might be less "powerful" than any other....Joseph no longer needed seer stones to see visions, and had grown in his abilities of seership.
To me, this explanation alludes to the notion that there is not and never was anything special about the rock JS used. It was merely a prop that JS could use to focus his pre-existent faith, a conduit through which he could learn to receive revelation.
This idea is strengthened by the fact the while we still have the "less powerful" brown seer stone (and supposedly other, more power stones) in our possession, none of the current prophets, revelators, or seers use them. If something like a seer stone actually worked, why wouldn't we use it now? Surely some of our modern church leaders lack the super faith of Joseph Smith. Wouldn't a seer stone help them fine tune their revelatory abilities as it had done before? Regardless, modern day seer stones are not used. They are recognized as simple rocks. Some may have fascinating geological histories, but they are still just rocks.
Now back to JS and Harris' counterfeit. If the brown seer stone were merely a prop which allowed JS to focus his faith and there was in fact nothing special about the particular brown seer stone outside of JS's belief that it was special, then why would a false stone make any difference? Joseph, firmly believing he had the correct rock, would have still focused his faith and translated. The only explanation then is that God (or somebody) really truly and in actuality has imbued certain rocks with special abilities. And if that's the case, why the hell aren't those super special magic rocks the most sought-after objects on the planet?
Sorry, this turned out waaay to long to cough out a simple rant. Thanks for listening!