Hi Andromeda, I'm not sure when the Jewellery Factory was introduced, but it seems to have gone the same way as the Mint. Not so useful as the devs initially thought it would/should be.
There was a time when we could choose to upgrade factories & trains by choosing the decrease in time only. That meant for the higher level players, the mine became less useful, as everything was upgraded quite quickly, so some could choose to make their own jewellery if they weren't buying it from the dealer.
In my main game, I bought it once I had no further upgrading to do. In my lower game, I bought it once the jewellery requirements were changed from ingots to ore. I still had factories to upgrade, but found it useful when I had a surplus of ore to make jewellery here and there. I probably only use it once or twice a month.
When, and which factories to buy, are pretty much our own decisions to make. I've never bought factories or community buildings as soon as they become available, and still don't have them all.