Questions & Answers about Yo no compraría una joya tan cara.
How do we form the conditional tense in compraría?
Compraría is in the conditional tense, expressing what you 'would' do. To form it for regular verbs, you take the full infinitive (comprar) and add the ending -ía. Because the yo (I) and él/ella/usted forms share this exact same ending, the pronoun yo is included here to make it perfectly clear who would not buy it.
Why is tan used here to mean 'such an'? Doesn't it mean 'so'?
You are right that tan usually means 'so' (as in 'so expensive' or tan cara). However, when English says 'such a [adjective] [noun]', Spanish changes the word order to 'un/una [noun] tan [adjective]'. Literally, una joya tan cara means 'a jewel so expensive', which is the natural way to say 'such an expensive jewel' in Spanish.
Is it possible to change the word order and say una tan cara joya?
Can I use the word joya to talk about jewelry in general?
Joya specifically means a single 'jewel' or 'piece of jewelry'. If you want to talk about 'jewelry' as a general category or uncountable concept, you should use the word joyería, which is also the word for a jewelry store.