Learn Farsi: ارزیاب این گوهر را کدر پنداشت.

This sentence uses a predicative complement. Here, in gowhar (this gem) is the definite direct object, so it takes the object marker râ. The word kedar (dull) is the complement describing the gem's state. With verbs of consideration like pendâshtan (to consider or suppose), the complement adjective or bare noun comes right after râ and immediately before the verb.

Why not just say in gowhar-e kedar râ using the ezafe?

If you connected them with an ezafe to make in gowhar-e kedar râ pendâsht, the meaning would change entirely. It would mean the appraiser simply supposed or imagined 'this dull gem' as a single object. By dropping the ezafe and separating the noun and adjective with râ, you show that 'dull' is the actual conclusion the appraiser reached about the gem.

What is the difference between using pendâshtan here compared to another verb of consideration like dânestan?

While both work perfectly with this grammar structure, pendâshtan carries a stronger nuance of subjective opinion, assumption, or even an incorrect assumption. It leans toward 'supposed' or 'imagined.' dânestan (to know/consider) sounds a bit more objective or factual. Since an appraiser is giving a subjective valuation that could potentially be debated, pendâsht captures that perfectly.

How widely is the word kedar used? Can I use it to mean a 'dull' movie or book?