The following comment is is not for you who know how pearls are formed. I just hope that by repeating the truth in various different wordings every chance I get, I can actually teach a lot of people how pearls are formed and make a dent in that urban myth of a grain of sand.
Let's take that goal to the next level. PGers not encumbered by business ties/responsibilities attend similar forums in future decked as collectors and aficionados. One mention of this sand business and we shriek like Victorians in a house of ill repute and feign a dead swoon. Bonus points if someone will follow us around with a chaise lounge on casters. Those who pretend not to be acquainted with us shake their heads sadly and say, "Poor dears. Allergic to urban mythology, and I hear Claritin only helps half the time...."
I think we'd help facilitate a bit of awareness-raising before we were banned for life... Maybe.
The shriek is a good idea if they don't have a Q & A period after a presentation. When someone comes out of the fake swoon, they'll have everyone's attention and it becomes a teaching moment.....
Even if I didn't have a chance to publicly humiliate a speaker for giving voice to that entrenched myth, I would certainly approach him/her when s/he is off the podium and set them straight.