Rubies, Ritual Table, and the Nether

Rubies: The Cursed Gem

Rubies were originally going to be Minecraft's trading currency back in Java 1.3.1. Jeb is colorblind and kept confusing them with redstone, so they got swapped for emeralds. The texture and pottery sherd are still in the game file (Prize) depicting a ruby.

Basalt Biome

In real life, rubies form alongside basalt. So now they generate exclusively in the Nether's (, where Redstone doesn't generate) basalt deltas, so no more colorblind issues, and basalt deltas finally have a reason to be visited.

Archeology

Suspicious gravel can also yield a ruby occasionally through archaeology.

Ruby Armor

Ruby armor fills the long empty gap between iron and diamond. Rubies are fire resistant, withstanding temps up to 2000°C and should survive lava drops, while diamond burns at around 900°C and now takes a stat debuff in the Nether unless the wearer has fire resistance. Ruby matches or slightly outperforms debuffed diamond in the Nether but stays below diamond in the Overworld so diamond stays relevant, alongside with Netherite Crafting being exclusive to Daimond.

Why We Shouldn't Have Emerald Armor

One may say, if we have Ruby armor, why not emerald armor? Emeralds are rated poor for toughness in real life and chip easily due to internal structure. Ruby has excellent toughness, beating even diamond on impact resistance.

Curses at the Enchanting Table

Using rubies instead of lapis at the enchanting table applies curses instead of regular enchantments. One of the new curses, the Curse of Pacifism prevents an item from harming entities, letting you move or push NPCs without dealing damage. Curses should also be applied to more objects, such as maps etc.

The Ritual Table

A new workstation specifically for necromancy. The Ritual Table can resurrect dead pets, returning horses as zombie or skeleton variants and wolves as new Bone Wolves, but only at night. Place either bones (skeletal variant) or rotten flesh (zombie variant), pay a ruby and an XP cost. A menu lists all your previously deceased pets by name or default identifier (like "Wolf #1"). Pets you killed yourself don't appear in the menu. New uses for bones and rotten flesh.

Other Stuff

A held ruby glows brighter when hostile mobs are nearby, echoing folklore where rubies warned their wearers of danger. Useful for spotting creepers before they spot you if you have it in your offhand.

Nether Pearl

Combining a ruby with an Ender Pearl creates a Nether Pearl, which points the way to Nether Fortresses in the Nether and ruined portals in the Overworld. A new Nether Conduit, crafted with a Nether Pearl and Magma Cream, provides fire resistance in a radius and lets you build under lava bases.

Why the Nether?

The Wildfire, a cut mob has a crown with a red jewel in it that looks suspiciously like a Ruby. I didn't include it as as suggestion in the post as to not break the rule, however, it was the basis for it being in the Nether.