Rare earth miner and processor MP Materials (MP -4.88%) had quite the week to remember, both operationally and on the stock market. On the back of some quite favorable news, the company's shares raced more than 16% higher over the course of the week, according to data compiled by S&P Global Market Intelligence.

A rare business

MP Materials owns and operates the only rare earth mine in the U.S. Rare earths, for those unfamiliar, are metals that are used in the manufacture of specialty magnets used in popular high-tech products such as smartphones.

As the only native rare earth miner and processor on our shores, MP Materials has attracted much attention in the current tariff war, which has a U.S.-China standoff at its center. The Asian country has by far the highest reserves of rare earths, and last week it halted export of these materials.

Just before the start of the trading week, the Financial Times cited "people familiar with the matter" as saying that the Trump administration was considering an executive order that would enable the stockpiling of rare earth materials.

That wasn't the only development in the saga. On Thursday, MP Materials said it had ceased shipping rare earth concentrate to China. This matters, because the company relies heavily on business partners in the country to process the materials.

Volatility alert

In the press release announcing that move, MP Materials wrote that "Selling our valuable critical materials under 125% tariffs is neither commercially rational nor aligned with America's national interest."

It pointed out that much of its production ends up with customers outside of China, including domestic clients and those in nations such as Japan and South Korea. It also said it is continuing to produce and stockpile rare earths.

While the bullish reaction to the Financial Times article was understandable, investors need to realize this is a fluid and potentially volatile situation that could change fast. Given that, I'd avoid MP Materials stock for now.