Looking at the film, it would make sense. When they first go into the gold room, Riley looks down and then easily lifts a notebook size piece of gold into his knapsack, which he then flings onto his shoulder. A piece that size would weigh quite a bit, he would have a difficult time just lifting it and his knapsack should break under such weight. Still, dog gone it, you just expect the hero stuff to be nice metal and weighty. At least I do.
I am getting some of the other nice gifts that came with the opening, so I will pop together a photo certificate to that effect for those few that are getting the coins on the JY thread. If they are plastic, I do intend to take advantage of the link above and fill that very expensive NT chest with as many of those coins for my own display if anything else. Will go along well with the PoTC chest I have of the Disney plastic coins and jewels mixed with the MR metal coins I have on display at home.