Silver prices surged above $40 an ounce Friday and theiShares Silver Trust (NYSE: SLV) closed with a nearly 3% gain. However, some analysts are cautioning investors not to jump in blindly.

Bullion holdings in the silver exchange-traded fund (ETF) rose to a record this week of nearly 360 million ounces amid the rally.

Despite the silver ETF's "seemingly relentless rise, conditions are not overbought, the ETF Review newsletter from Investors Intelligence said Friday.

"However, trading is very overextended relative to the moving averages, so a reversion to the mean will occur eventually," the newsletter stated. "When that pullback comes it is likely to be harsh and swift. As such we are reluctant to chase this momentum trade. It's too late in the game and we would only now buy on a pullback to moving average support."

The rally marked the ninth day this year in which the silver price reached a new 31-year high. The only record remaining now for the silver price is its all-time high above $50 per ounce, set in 1980 as the Hunt brothers attempted to corner the market.

With today's advance, silver extended its year-to-date gain to about 30%, far surpassing that of almost any other commodity. Moreover, the price of silver has now doubled since September 2010.

ETFS Physical Silver Shares (NYSE: SIVR) is another ETF that holds silver.

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