Like the Dead Sea Scrolls and the Nag Hammadi Library, The Book of Mormon was stored away for over a millennia. To make sure the record would not deteriorate over time, the record keepers used gold or an alloy of gold to engrave their writings upon.

Though the word "plates" was often used to describe the record back in 1830, a better word was used by Joseph's wife Emma to correctly describe the nature of the record - thick paper:

I once felt of the plates, as they thus lay on the table, tracing their outline and shape. They seemed to be pliable like thick paper, and would rustle with a metalic sound when the edges were moved by the thumb, as one does sometimes thumbs the edges of a book. (Vogel, Early Mormon Documents 1:541)

 

The size of the plates was described by Book of Mormon witness Martin Harris:

The plates were seven inches wide by eight inches in length...they were altogether about four inches thick.

I hefted those plates many times, and should think they weighed forty or fifty pounds. (Tiffany's Monthly, August 1859, 4:163)

Joseph's neighbor (Mr. Beman) who made a box for them said he heard them "jink" when they were placed in the box. (Ibid.)