Radiant Ore is one of the trickier materials to access in Slime Rancher 2, and it'll require you to venture a little further out of the way in order to harvest it. Still, since it's one of the components needed to craft the jetpack, I'd say it's well worth it, especially as this will let you explore even more areas and procure rarer slime friends.
The jetpack is also required to gather Moondew Nectar—one of the game's most coveted resources. Though you will need to craft an upgrade to get Radiant Ore, you can do it right at the start of the game. Here, I'll explain the step-by-step of where to find Radiant Ore, and how to access the area so you can harvest it for yourself.
How to craft the Resource Harvester
In order to gather Radiant Ore, you're first going to need to craft the Resource Harvester upgrade using the fabricator in the cave underneath the conservatory. To make this, you require 450 Newbucks and ten Cotton Plorts. You can find Cotton slimes just to the west of the conservatory. Feed them veggies and they'll release some plorts you can gather, though you may want to put them in a pen to make the process easier.
Newbucks, on the other hand, come from selling plorts to the market at the back of the conservatory. You can see what each plort is worth on the board, and to sell them, you just fire them at the market. Once you've got everything, yeet your ten Cotton Plorts into the refinery near the fabricator and then craft the upgrade. Now you're ready to get some Radiant Ore.
Where to find Radiant Ore
The best place to find Radiant Ore early on is Ember Valley—the westernmost island in the game's world. Head west out of the conservatory and keep going until you find the Pink Gordo slime shown on the map above. Feed this hungry boi thirty vegetables and it'll explode, unlocking a geyser that will lift you into the air if you stand on it. Jump up onto the left platform, run along to activate the warp point for Ember Valley, and head there yourself.
Radiant Ore seems to randomly spawn throughout Ember Valley, but each outcrop usually gives you two pieces, meaning you only need to find five outcrops to get enough to craft the jetpack. While there, also make sure to grab those required Phosphor Plorts, since there are lots of Phosphor slimes in this area. When you've gathered everything, head back to the refinery, fire in the resources, and build that fancy jetpack.
Once you have the jetpack, you'll find Radiant Ore in other areas such as high up in the Starlight Strand, so be sure to keep an eye out.