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) shot up as much as 12% in intraday trading on heavier-than-average volume.

So what: When it comes to prices for rare earth metals, the math is simple for Molycorp: Higher prices mean a brighter outlook. So there should be little surprise that investors are getting excited about the stock after Reuters reported that a tonne of rare earths now goes for more than $100,000. While that may sound like a pretty penny on its own, consider that the price is up from $14,405 per tonne last July. Rare Element Resources (NYSE: REE) and Avalon Rare Metals (NYSE: AVL) were also both up sharply on the news, while the Market Vectors Rare Earth/Strategic Metals ETF (NYSE: REMX) posted a more modest gain.

Now what: There was perhaps a trace of egg on Jim Cramer's face as he was in the headlines yesterday for reiterating a "sell" call on Molycorp. But frankly, I can't help but agree with him. Molycorp has been on a monumental tear since its July 2010 IPO and now trades at more than triple its IPO price. Rare earths are a hot commodity right now, but as many great investors have noted, the time to buy a stock is generally when nobody's talking about it or the outlook appears bleak, not when investors are excited and brimming with confidence.

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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 prefers dividends over a sharp stick in the eye.