Learn Arabic (Modern Standard): اشترى الصائغ كل الذهب ما عدا ذهب عيار عشرين. - The jeweler bought all the gold except twenty-karat gold.

ما عدا is another very common way to say 'except' in Modern Standard Arabic. While إلا is a particle, ما عدا actually functions as a fixed verb meaning 'it excluded' or 'passed over'. Because of this, the noun that follows it is grammatically treated as its direct object.

Why is the word for twenty عشرين here instead of عشرون?

It is in the genitive case because it is part of an idafa (possession chain) with the word before it. In Arabic, the 'tens' numbers (20 to 90) decline like regular masculine plurals: they end in -ūn for the nominative case (like عشرون), but change to -īn (like عشرين) for the accusative and genitive cases.

How does the phrase ذهب عيار عشرين work grammatically? Why aren't there any adjectives?

This is a three-part idafa (noun chain) used to express material grades. Literally, it translates to 'gold of the measure of twenty'. Because it is an idafa linking nouns together to define one another, the first two words (ذهب and عيار) cannot take the definite article ال.

If I wanted to say 'every piece of gold' instead of 'all the gold', how would I change كل الذهب?

You would simply remove the ال and say كل ذهب. When كل is followed by a definite noun, it means 'all of the' item. When it is followed by an indefinite singular noun, it changes meaning to 'every' item.