gems --- countable or uncountable?

[th.19]

Hello teachers! Good morning from Japan.

I have a question about the usage of the countable / uncountable
form of gemstones (diamond, opal, etc...).

When I look up these words in the dictionary, it says that
they can be used both as a countable and uncountable
noun.

What exactly is the difference between the two, and if I were
to use it in the following sentence, which should I use?

"People started to live in the town after they found some
opal(s) in a cave nearby."

Thank you for your help as always!

[J&K Tutoring]

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...'

[SoothingDave]

Opal | Define Opal at Dictionary.com

I disagree. I think opal is like coal, always singular except when discussing different varieties. In the dictionary, "opal" has no plural form given.

[Raymott]

You could use the mass noun in the original sentence.
Opal was found in the cave. It was dug up, cut up and polished, and some opals were made out of it. A mass of opal in the earth is opal. A jewel made from this is an opal.

[SoothingDave]

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...'

I wonder what was living in the town before people started living there.

[Tdol]

You could use the mass noun in the original sentence.
Opal was found in the cave. It was dug up, cut up and polished, and some opals were made out of it. A mass of opal in the earth is opal. A jewel made from this is an opal.

That's the pattern I would use.

[th.19]

Sorry teachers for the late reply --- and thank you all for your comments.

The idea of "mass noun" makes a lot of sense. I will be using the singular tense in the example that I cited.

Once again, thank you all for your helpful answers!