Adam Clarke on Deuteronomy 8:9

A land whose stones are iron - Not only meaning that there were iron mines throughout the land, but that the loose stones were strongly impregnated with iron, ores of this metal (the most useful of all the products of the mineral kingdom) being every where in great plenty.

Out of whose hills thou mayest dig brass - As there is no such thing in nature as a brass mine, the word נחשת nechosheth should be translated copper; of which, by the addition of the lapis calaminaris, brass is made. See on Exo 25:3 (note).