• Will the mod be ported to Forge?

We are thinking of moving to Forge if the mod size keeps expanding at this rate.

• Will there ever be some kind of 'guide book' to help players understand the gems?

Recipes can now be viewed using REI, which is a soft-dependency going forward. Gem loottable viewing will be possible in an upcoming update (1.5 >)

• Will the mod ever be backported to < 1.19.4?

No, hard no.

Overworld

• Petrinite: a gem that is held together by roots and flowers around its location, found in swamps and lush caves.
• Antholium: a gem that forms only in the warmest of ocean climates. usually surrounded by coral.
• Spironite: a cold gem, only generating in the coldest of biomes, or the highest of mountains.
• Dyanite: A shiny gem that can be found in and around the deepslate layers.
• Ruby: a gem that can be found underground.
• Cobgalite: a gem that is easily mistaken as a rock, can be found in the deepslate layers.
• Sculk chitin: a mass of chitin and sculk, only found in the deep dark.
• Amalgamite: a mass of ores bundled together into one, can be found in the stone layer.

Nether

• Slagodite: a gem that can be found in nether wastes, crimson forests and basalt deltas!
• Jadestone: a warped gem that can only be found in warped forests.
• Biolium: a gem that can be found inside the crimson forests and wastes, held together by biological material.
• Runian: a gem formed by souls inside the soul sand valleys, you can kill a mob on top of soul sand or soil for a chance to create this ore!

End

• Denarium: a gem which has unknown origins, can be found on the outer end islands
• Glazonium: a gem filled with beautiful Prismarine, can be found on the outer end islands.