Translation:The ruby is a beautiful mineral.
Mmm for me, πέτρα is a bit too unspecific. Πέτρα is a direct translation to stone, and even in English, the ruby is considered a precious stone. But the word πετράδι could do. I'm not sure why, but it's used almost exclusively for precious stones and jewerly, even without the adjective πολύτιμο. ^.^
"πολύτιμος λίθος" is the most accurate translation of "precious stone", but if you say only "λίθος" it's just a stone. An archaic stone, but a stone.
ορυκτό = mineral. I think (but I am not a geologist) ore is a metal mineral only, hence ruby is not an ore
(Miner here). You're right. But it's just a naming convention. The difference is subtle. Ruby has Aluminium in it (Al2O3). So, technically, it has metal. But it's not excavated for its aluminium. That's why it's not classified as an ore.
I'm not sure if this is a difference every language makes, though.
Oh thanks! Nice to know! My only suggestion here is to consider both "ore" and "mineral" as acceptable answers.
This is out of my comfort zone, as I am not very familiar with geology, but to briefly explain the linguistics of this question:
"ορυκτό" comes from the ancient greek word "ὀρύσσω" which means "dig", so, broadly speaking, it's something you dig out of earth. The closest translation is mineral. A mineral that produces metal is called "μετάλλευμα".