Apple continues to defy gravity, stock hits $600 NEW YORK (Reuters) - Every couple of weeks it's nice to look back and see just how far Apple has come. Shares of the iPad and iPhone maker eclipsed the $600 barrier for the first time on Thursday, only about a month after topping the $500 mark for the first time. Just a day before Apple's new iPad becomes available in stores the stock ticked up to $600.01 at Thursday's open before selling off. The new iPad, at a starting retail price of $499 is cheap compared with one share of stock. Apple's rally has been stunning - the stock is up 47 percent for the year and nearly 10 percent for the month. Last month, the company became only the sixth company in U.S. history to top $500 billion in market capitalization, a significant move for a company whose stock traded at $10 about a decade ago. None of the others, which include Exxon Mobil and Microsoft, were able to sustain that market value over time. Earlier this week, Morgan Stanley and Canaccord Genuity both lifted their price targets on the stock to above $700. Apple currently makes up about 18.5 percent of the Nasdaq 100 stock index. It was cut to 12.3 percent from 20.5 percent in April 2011, but a surge in price has pushed the stock back up, making this index of 100 well-known companies hostage to the performance of a few technology titans like Apple. One out of every five stock options trades in Wednesday's options market was in Apple, according to Henry Schwartz, president of options analytics firm Trade Alert. Options trading volume hit a record 2.11 million contracts on Wednesday, according to Trade Alert. In all, 1.29 million call options and 812,000 put options changed hands in Apple on Wednesday, with the most active March calls at the $585, $580, $590 and $600 strikes, Trade Alert data showed. The at-the-money 30-day option implied volatility on the stock jumped 35 percent to 36 percent, suggesting traders are expecting much large price swings for shares. Reporting By Angela Moon; additional reporting by Doris Frankel in Chicago