Rubies...

[westkunint]

Where do you find rubies? I can't find any. Is it just bad luck?
Off topic: where do you find aglets, too?

[Fantastic Fwoosh]

Rubies are underground. You can find them hanging off walls or embedded in stone.

Typically they are down deep underground, and in ice biomes, gemstones hang out of the walls visibly.

Aglets can be found in chests, keep searching and try looking in some chests within the jungle for good loot.

[Bomb Bloke]

Another method of getting rubies (and the various other gems) is to obtain an Extractinator (most often found in gold chests), then pour your reserves of silt etc into that. There's about a 1/300 chance of getting a ruby out of any given "rubbish" block, so if you want a large quantity you'd be better off mining them normally.

Still, the other goodies the Extractinator gives you make it well worth hunting down. For example, it's also the only way to get amber.