Learn Farsi: به دلیل ناخالصی‌ها، ارزیاب آن یاقوت را کدر شمرد. - Because of the impurities, the appraiser reckoned that ruby dull.

This sentence uses a predicative complement. If we said ân yâqut-e keder râ, it would mean 'the dull ruby' as a single noun phrase, like if someone just bought a dull ruby. By placing keder immediately after râ, it acts as the complement to the verb shomord. It shows the result of the evaluation: he considered the ruby (direct object) to be dull (complement).

I originally learned shomordan as 'to count'. Is it common to use it to mean 'reckon' or 'consider'?

Yes, especially in formal or written Farsi. Just like dânestan (to know) can mean 'to consider', shomordan (to count) is frequently used to mean 'to reckon' or 'to evaluate'. When used this way, it almost always takes this exact structure: a direct object marked by râ followed by an adjective or noun complement.

How is the word nâ-khâles-i-hâ (impurities) formed?

It is built systematically from the root adjective khâles (pure). Adding the Persian negative prefix nâ- makes nâ-khâles (impure). The suffix -i turns that adjective into the abstract noun nâ-khâles-i (impurity). Finally, -hâ makes it plural.

What exactly does the adjective keder describe in this context?