There really is no best time - it's just where you get fossils from. You can only get fossils from the underground where you dig them out. You get the underground/explorer kit from the man in Eterna City. Go underground in different areas and dig a lot. Soon, you will have lots of different fossils. You can regenerate them in Oreburgh City - the man at the desk in the museum will help you. Good Luck!
You have to dig them up when you're underground
go underground
dig until u get them........
Dig Underground
You can find fossils in the Underground Tunnels, or underground. Just dig for them as you would spheres.
fossils, shards, stones, plates, spheres
you dig for fossils and you can get aerodactyl shieldon omantye kabuto lileep
You can dig up fossils underground by using the Explorer Kit in the Key items pocket of your bag :)
No, but you can dig up fossils and gyroids, and sell them for bells. -TB
I'm not sure exactly if you can get it in diamond..... But if you can, you have to dig it up underground
Use the Explorer kit and go underground to dig into the walls, you will find fossils, spheres, shards and other items