Jim Cramer -- Why Is Janet Yellen Suddenly a Stock Commentator?

NEW YORK (Real Money) -- Thanks for nothing, Janet Yellen. Yep, the Fed chief used some pretty frightening terms about stocks Wednesday, saying that valuations "generally are quite high" and that there are "potential dangers" with them.

Now everyone's entitled to an opinion, but that doesn't mean I think everyone should give one, especially the Federal Reserve chief.

What makes me write that? History.

Consider that many investors were freaked out of stocks when in December 1996 then Fed Chairman Alan Greenspan talked about how stocks might be suffering from "irrational exuberance." The Dow happened to be at 6,308 then; it's over 17,800 now. The S&P 500 was at 726; it's now at 2,081.

Or how about in July of last year when Yellen herself said that valuation metrics in some sectors including biotech and social media "do appear substantially stretched."

At the time, the biotech index was at 2,750; it's now at 3,853. Regeneron (REGN) was at $317; it's now at $474. Gilead (GILD) stood at $83; it's now at $102. Biogen's (BIIB) scooted from $319 to $385.

Some stocks like Pharmacylics (PCYC) were especially stretched at $98, but then Johnson & Johnson (JNJ) and AbbVie (ABBV) got in a bidding war for it, and the stock's now at $255. Synageva (GEVA) weighed in at $78, but Alexia just agreed this morning to pay $230 a share for it, fully 116% above yesterday's price.

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