Though not very valuable, one of the world's great coal reservoirs is located to the southwest. To the northwest, Michigan and Minnesota are noted producers of iron ore, while Wisconsin's nickname, the Badger State , is a reference to early lead miners. Still farther south is Arkansas, the Quartz State and long the only state known to harbor diamonds. Vast beds of limestone and marble lie across much of the Midwest and South. Both are quarried for use as building stone.
Oil is the most valuable mineral resource in Texas and neighboring Oklahoma and Louisiana.
Nevada is nicknamed the Silver State , Arizona the Copper State , though neighboring Utah now produces more copper. These days, fossil fuels are major sources of revenue in Montana, Wyoming and Utah. Colorado celebrates its gold and silver lodes on its state flag. Long known as the Copper State , Arizona even boasts a copper-colored star on its state flag, though neighboring Utah now produces more copper. Nevada is nicknamed the Silver State .