Metals

Because Gold is used as a symbol for purity, value, royalty.
see below :
Gold has been used as a symbol for purity, value, royalty, and particularly roles that combine these properties.

Also, gold was very common in Egypt. See below :
Egyptian hieroglyphs from as early as 2600 BCE describe gold, which king Tushratta of the Mitanni claimed was as "common as dust" in Egypt.

Therefore Egyptian kings use gold as the cover of their coffins