The plural form should be used in the example cited.
"People started to live in the town after they found some
opals in a cave nearby." (countable) 'A man made an opal necklace from what he found.' (uncountable)
That takes care of the word opal. Now for the other problem in your example: "People started to live in the town..." As written, it sounds as if the people were in the town- but not living- before the opals were fround. Better to say something like, 'People came to live in the town after...' or 'People started moving to the town after...'
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