Learn Croatian: Smaragd ima bolju čistoću nego što sam mislio. - The emerald has better clarity than I thought.

We use nego što because we are comparing a noun phrase with a factual statement (sam mislio). You use nego da when comparing hypothetical actions. Because 'what I thought' is a real fact that happened in the past, nego što followed by the indicative tense is required.

Could I use the preposition od here instead of nego što?

You cannot use od directly before a verb clause. However, you could expand it and say od onoga što sam mislio (than that which I thought). Both are grammatically correct, but nego što is much shorter and more natural for everyday speech.

I learned that čistoća means cleanliness. Is it the normal word for a gem's clarity?

Yes! While you usually see čistoća used for cleanliness or purity in everyday contexts, it is the standard vocabulary word in Croatian for the clarity of gemstones like emeralds and diamonds.

Quick refresher: why is the phrase bolju čistoću and not bolja čistoća?

The phrase is the direct object of the verb ima (has), so it must be in the accusative case. For feminine nouns ending in -a like čistoća, both the adjective and the noun take a -u ending in the accusative, giving us bolju čistoću.