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I didn't say it was for decoration. I'm talking about a service, i.e., maintaining these creatures in the city. Moreover, the concept of "a lot" is vague. If ore arrives, let's say, in tons, then rare resources are mined in much smaller quantities (for example, 1 gram of gems can be as valuable as 2 tons of ore).
Still, you can make an entire pool of mercury for dragons this way. And do devils feed on this mercury? And do angels make necklaces out of gems?