Olivine Ore is an addition from GregTech. It spawns in some places in The End.
Olivine ore is found in The End, usually on asteroids. It needs to be mined with a diamond pickaxe.
Olivine ore is the fastest way to obtain Olivines and Olivine Dust. With Industrial Grinder, getting emeralds by grinding olivine ore is generally faster than trying to mine Emerald Ore in the mountain biomes.*
Unless you have like, 8 emeralds.