The True Polymorph spell allows you to turn a creature into an object and if you concentrate on it for 1 hour it becomes permanent. The relevant text follows:
Choose one creature or non-magical object that you can see within
range. You transform […] the
creature into an object […].[…]
Creature into Object. If you turn a creature into an object, it
transforms along with whatever it is wearing and carrying into that
form. The creature's statistics become those of the object…
Can polymorph be used to create magic items by polymorphing creatures into objects? Answers can have DM Fiat, but I would prefer a rules based answer or an answer by Jeremy Crawford or Sage Advice or some other 'official' source.
Best Answer
You cannot create magic items with true polymorph
Per the 2018 PHB errata, true polymorph now explicitly disallows the transformation of creatures into magic items:
Rules as Intended agrees
So Jeremy Crawford, lead rules developer of D&D, had ruled previously about the intent (which now agrees with the RAW above):
he also indicated this previously in a tweet: