Luminous Alloy is a crafting component in God of War Ragnarok that is commonly used for crafting upgrades for shields. It is an uncommon resource and if you do not know where to get it, there is a good chance you will get through a large portion of the game without being able to purchase upgrades for your shield.
You may have noticed a lot of those barking lizards around the game. Sometimes it’s obvious how to kill them and other times it is not. You will need to attack them from behind in order to kill them. When you do, they will usually drop a few pieces of luminous alloy that you can use for your shield upgrades.
When it comes to upgrading your shield, you should try to focus on one single shield to purchase the upgrades for. You are not going to have an abundance of luminous alloy to spend upgrading lots of different shields. Since they all provide the same amount of stats, pick the one you like the look of most and stick with this for the upgrades as it will allow you to get to a much higher level without the trouble of running out of alloys to use for the upgrade.