Occam’s Razor would suggest that when someone emptied their pockets of electronics as required, the bag was accidentally included or purposely ditched out of fear.
Cocaine is the drug of choice for elites. $100/gram or more. Could have been anyone of the guests. If it was intended for use at the party, it would have required some privacy since it was a rock. Would have needed a surface, a credit card or something to cut it and a rolled up hundy. This suggests it was someone that had an expectation of being able to have this privacy in the White House - an insider.
It was originally reported as a white powder, not a rock.