It's time to make Sprint Nextel your next big spec play, Jim Cramer said Friday on CNBC's "Mad Money." In light of recent events, he's calling Sprint the "Holy Grail of speculation."

Previously, Sprint has been nothing more than the "perennial third-wheel" in the U.S. wireless business and the company has struggled for years. So, why is Cramer so sure it's going up and not down?

Whenever you have a stock trading below $5 per share and it breaks through that level, he said, you know you're getting a real turnaround. Cramer noted the $5 mark was important, because it opens up the door to a whole new class of buyers — most big mutual funds and hedge funds aren't allowed to own stocks trading under $5. With more buyers in the mix, stocks tend to go even higher, he said. That's why he likes Sprint right now at around $4.30 a share — roughly 70 cents away from that all-important threshold.

Sprint also reported a "phenomenal" second quarter Wednesday night, which Cramer said further solidified the firm's comeback story as one of the greatest out there. Over the last six months, Sprint's share price has doubled and the stock surged 20 percent on Thursday alone. But despite all this, Cramer urged anyone who owns Sprint not to ring the register just yet.

Before, the stock was a trade, he said. "Now I think it's an investment."

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