Kuranyi, hoping for a Germany recall this World Cup year, grabbed the winner three minutes after the restart, tapping in Jefferson Farfan's cross from the right.
Leverkusen top the table on 38 points after 18 matches following their 4-2 win over Mainz on Saturday. Bayern Munich are third on 36 points after beating Hoffenheim 2-0 on Friday.
A stoppage-time goal from Borussia Dortmund's Kevin Grosskreutz gave them a 3-2 win over hosts Cologne, who battled back from 2-0 down to equalise two minutes from time. The win kept Dortmund in the mix in fifth place on 33 points.
"I do not evaluate this after only 90 minutes but if he scores in every match then I am happy to have kept him," said Schalke coach Felix Magath, who wanted to sell Kuranyi with Sunderland offering a reported 3.4 million euro deal which the striker turned down during the winter break.
"I never thought about leaving the club," Kuranyi said. "So it was not hard for me to focus solely on the match. I was there where a striker should be and I scored."
Nuremberg, a place off the bottom with 12 points, troubled the hosts with a new-look defence including Breno, on loan from Bayern Munich, and twice hit the post.
The visitors were initially in control as Schalke's offensive efforts were stifled and Nuremberg could have taken the lead when Angelos Haristeas pounced on defender Marcelo Bordon's fumble to race clear but hit the post.
Schalke, who have now won their last four league games, grabbed the winner in the 48th minute when Peruvian Farfan shook off his marker on the right wing to cross into the box and Kuranyi stretched to connect at the far post.
But Nuremberg, with coach Dieter Hecking in his first match in charge, refused to give up.
They were again unlucky in the 68th minute when Greece international Haristeas combined well with substitute Christian Eigler, who sprinted clear but had his shot from 10 metres deflected on to the post by keeper Manuel Neuer.
