These metals and elements have become indispensable for many advanced industries, but why are they so important?

And what is their geopolitical role when China and the United States are at loggerheads on trade?

- What are rare earths? -

Rare earth elements (REEs) are 17 metals that are actually not that hard to find, with some as abundant in the planet's crust as lead or copper.

Their "rarity" is a result of it being hard to find deposits that are commercially viable. The metals are only found in small quantities within various ores and are often hard to extract.

The United States Geological Survey regularly searches for deposits that can be exploited commercially without damaging the environment.

"Most of the REEs are hosted by minerals that have complex chemical formulas; this presents more challenges to process and extract the REEs," says the USGS.

- What are they used for? -

Rare earths have a particular atomic structure that gives them unique physical properties.

Europium, for example, has a red luminescence used in television screens. Neodymium, which is naturally magnetic, is used to make powerful mini-magnets.

Lanthanum is used in rechargeable batteries found in many electric and hybrid cars.

According to a report published by the British Geological Survey, "REEs are used in the widest range of consumer products of any group of elements".

"REEs play a vital role in environmental protection, improving energy efficiency and enabling digital technology," it added.

- Where are they found? -

According to the USGS, China has the world's largest rare earth deposits, with 44 million tonnes of reserves.

Vietnam and Brazil have 22 million tonnes each.

China has two advantages. Its ores are buried in clay deposits -- unusual but favourable, as it makes them easier to extract, says the USGS.

Chinese environmental standards are also less strict than in the United States, the institute says.

In contrast, difficulties with US deposits twice forced the closure of the only US mine, in Mountain Pass, California.

"Production at Mountain Pass has resumed in the first quarter of 2018," noted commodity specialists Cyclope in a report, before adding that "a large part was destined for export to China."

"There's no refining capacity in the world outside of China," James Litinsky, co-chairman of the Mountain Pass mine, told US business news TV channel CNBC, though he believes the facility will be self-sufficient from China and able to produce its own separated rare earth products from 2020.

- Why are they strategically important? -

The lack of processing sites outside China, plus its production capacity, makes Beijing the main player in the rare earths market.

China's pre-eminence in the supply chain of these metals is a nightmare for the US, as its high-tech companies, both civilian and military, rely heavily on rare earths.

The situation is further strained by the trade conflict between Washington and Beijing.

The issue hardened at the end of May, when Chinese President Xi Jinping visited a rare earth factory, dangling the threat of cutting exports to the United States as a counter-strike in the trade war.

China has used the tactic in the past, as in 2010 when Beijing abruptly halted rare earth exports to Japan in retaliation for a territorial dispute.