Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of rare earths miner Molycorp (NYSE: MCP) were up more than 13% in intraday trading today on reports that Japan's Sumitomo may buy a stake in the company.

So what: Rare earths have been in the spotlight lately as China -- which provides roughly 97% of the world's supply of these obscure minerals -- sparked concerns by showing a willingness to seriously clamp down on the amount it's willing to export. The worries have jazzed up investors and launched the few publicly traded rare earths stocks into the stratosphere. It's not that surprising then that not only Molycorp, but Rare Element Resources (NYSE: REE) and (to a lesser extent) Market Vectors Rare Earth ETF (NYSE: REMX) are up today on the Sumitomo rumors.

Now what: Rare earths stocks have been launched into a distinctly bubblish orbit and the vague news that a trading firm may be buying some as-yet-undefined stake doesn't change the underlying value of the company. I can get down with the occasional speculation, but with Molycorp we're talking about a stock that's got a lot of excitement around it, still has its hopes of success well out in the future, and could have its prospects radically altered by decisions from unpredictable Chinese officials. Sure, things could work out well for investors, but I think the view is pretty comfortable from the sidelines.

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Fool contributor Matt Koppenheffer does not own shares of any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy assures you no Wookiees were harmed in the making of this article.