Answer
Crystals are a currency/developer-product topic; avoid generator pages and use official or current guide sources.
Crystals are treated as a currency/developer-product topic for Piggy: Intercity. A reliable free farming route is not published as final on this site.
If you are looking for free Crystals, avoid generator pages and survey sites. Use official game UI, shop/currency pages, or current guide pages only.
If a future free route or reward source is confirmed, it should be listed with exact steps and route confidence.