Hades 2 has so many crafting reagents that even Melinoe often complains about them. G. Rock is one such reagent. Here is where you can find Glassrock in Hades 2.
Hades 2 How to get Glassrock and G-Rock Farm location
Before you can find Glassrock in Hades 2, you must first clear the second zone, Oceanus. Only after you have cleared the zone and defeated Scylla and the Sirens will you be able to access the next zone where you can find G.Rock spawns.
Make sure you have your Crescent Pick handy, or better yet, a trusted person who carries the Crescent Pick tool at all times.
You can find Glassrock in Hades 2 in the Fields of Mourning. Since this is a roguelike game, you can never predict when or if items will spawn on each run. It also doesn't help that Fields of Mourning has very large levels where it's easy to miss collectibles, so make sure you check every corner.