How can I replicate this underground "ore-vision" system in Unity? Something mask-related?

Here is an image of the masking effect I’m talking about: https://i.ytimg.com/vi/gnelWysPeKs/maxresdefault.jpg

I have so far used a (very) primitive spotlight system to hide the ore out of range, but I’m looking for something like this. What is this technique called? Not even sure what to google for.

The game is on Xbox 360 Indie Store and is called Miner Dig Deep.

Example gameplay video here: https://www.youtube.com/watch?v=8Mvy5D_wb80

Thanks!

This is a stencil buffer.