Tom Riddle's goal is to to make a horcrux from each house owner's belongings. He is successful in making a horcrux from three of the owners' items. Why isn't he successful in making the Sword of Gryffindor a horcrux?     “I can only guess,” said Dumbledore. “For the reasons I have already   given, I believe that Lord Voldemort would prefer objects that, in   themselves, have a certain grandeur. I have therefore trawled back   through Voldemort’s past to see if I can find evidence that such   artifacts have disappeared around him.”      “The locket!” said Harry loudly. “Hufflepuff’s cup!”      “Yes,” said Dumbledore, smiling, “I would be prepared to bet — perhaps   not my other hand — but a couple of fingers, that they became   Horcruxes three and four. The remaining two, assuming again that he   created a total of six, are more of a problem, but I will hazard a   guess that, having secured objects from Hufflepuff and Slytherin, he   set out to track down objects owned by Gryffindor or Ravenclaw. Four   objects from the four founders would, I am sure, have exerted a   powerful pull over Voldemort’s imagination. I cannot answer for   whether he ever managed to find anything of Ravenclaw’s. I am   confident, however, that the only known relic of Gryffindor remains   safe.”   Canon information from Pottermore, or the HP wikia, is acceptable.
Voldemort was said to be very interested in the four founders of Hogwarts. He created a Horcrux from the Ravenclaw diadem, the Hufflepuff cup, and the Slytherin locket. Did he try to find a Gryffindor relic to complete the set, or did he not bother?   I can't see him giving up on such a task, but maybe there was insufficient time.  Gryffindor's sword was hiding 'inside' the Sorting Hat, but a famous family usually has some other heirlooms too.  If there are no canon answers I'd be interested in your own speculation!
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