(UPDATED)Slime Rancher 2 Best Strange Diamond Farming Route(update in description)
Update: With the full release, drones have come. You can place a drone outside now. So go to the cave where the second diamond is in this video and place a quantum drone there. In a day of playing the drone found like 20 strange diamonds and sent them all to the quantum cloud.
I have found strange diamonds randomly all over the world but these two caves are the most consistent and sure way I found them. You can go a little slower than I am on this video and farm for other resources found on these route.