It is correct that the Rules as Written state Non-Magical Items are turned to stone. But personally I disagree with the assessment that nothing happens to Magical Items, it is unstated for the purpose that it is up to the Dungeon-Master. Two reasons.
- It is not stated. Unless a rule is explicitly stated... (ie. in the positive) (eg. The rule actually reads as follows "Magical Items are unaffected by petrification.") ...it is not a rule - leaving it up to interpretation. There is no stated ruling in the DMG on what happens to Magical Items upon petrification, only a hermeneutically interpreted position on petrified Magical Items. My feeling that it's written this way on purpose and for this reason.
- It's your table. The Dungeon Master has unilateral control of the rules at their table. In interpreting, adding, or omitting rules, the Dungeon Master always has the final say.
That said, to your 3 year old question lol. I agree with your ruling. It would create a strange precedent for breaking magical items in my games, for a petrified object is just stone and can be easily broken. But you can always make exceptions depending on power level and how much plot armor you need added to a specific magic item.
I simply wanted to address the notion that just because the DMG kind of says something, you have to do it a certain way. In my opinion, the DMG does not give a clear answer on this.
I hope your table isn't still waiting on a ruling for this. :). If so, roll!