I can't code this because i don't know how. so im just sharing my idea. NOTE: when i say gem, i mean emerald/diamond/ruby.

Gem mod Features:

Rubies (used for decoration, can be embedded into tools and armor)

Crowns (3 gold ingots, 1 gem, 2 gold nuggets, has emerald, diamond, and ruby variants, each with effects)

Crown effects: strength (ruby), resistance (diamond), hero of the village (emerald)

Gem glass (smelt a gem to get a transparent gem, 4 of those to the glass version)

Gem lamps (1 chiseled stone bricks, 1 gem, 2 glowstone dust, 1 redstone, emits a light level of 7 when powered)

yeah thats it

Oh wait!!!! VOLCANIC TUBES!!!!!

These would spawn in mostly oceans (but other biomes too, other than the flower forest and mushroom island) underground. They would be made of a mix of basalt and smooth basalt, and have an abundance of rubies, diamonds, and emeralds. They would have lava at the bottom and (if they surface) have a volcano structure at the top.