After three consecutive years of increases, the value of US mineral production declined last year.

According to the US Geological Survey's Mineral Commodity Summaries 2014 report, the US produced $74.3 billion worth of minerals in 2013, down slightly from $75.8 billion in 2012.

Net exports of mineral raw materials and old scrap contributed an additional $15.8 billion to the US economy, USGS wrote.

"To put this in context, the $90.1 billion value of combined mined, exported, and recycled raw materials is more than five times greater than the 2013 combined net revenues of Internet titans: Amazon, Facebook, Google, and Yahoo," USGS Mineral Resources Program Coordinator Larry Meinert said in a statement. "This illustrates the fundamental importance of mineral resources to the nation’s economy, technology, and national security."

This report, which the USGS produces each year, includes statistics on 90 mineral commodities.

The drop in value was not due to lower production – in fact, production increased for most industrial mineral commodities and remained stable for most metals (though domestic gold production declined by 3%). Rather, a drop in metals prices resulted in the reduced overall value of US mineral production.

Production from US metal mines resulted in an estimated value of $32 billion – an 8% decrease on the year before. Gold contributed 32% of that value and copper 29%.

"Minerals remain fundamental to the U.S. economy, contributing to the real gross domestic product at several levels, including mining, processing, and manufacturing finished products," the USGS wrote. "The U.S. continues to rely on foreign sources for raw and processed mineral materials."

Employment in the mining sector

The Mineral Commodity Summaries 2014 also provides statistics on employment in the mineral industry.

The number of production workers in coal mining dropped from 76,000 in 2012 to 73,000 last year.

Meanwhile, employment in the metal mining sector has sky-rocketed over the past five years. In 2009 there were 28,000 production workers in metal mining. Last year there were 105,000 – a 2,000-person increase on the year before.