Typically, minerals can easily be seen by the color of gem sticking out of black mining nodes in Disney Dreamlight Valley. Onyx is a black gemstone, however, and unfortunately there aren't even black gems to mark a mining node that with give you Onyx. Here's how we found it in Disney Dreamlight Valley.
How to get Onyx in Disney Dreamlight Valley
Onyx is a lot like Rich Soil, in that it isn't obtained by mining any nodes in particular, but by mining a lot of them. After mining about 20 nodes we were able to find our first stone. As indicated by the Collection Menu, it's not tied to any Biome in particular.
If you are out looking for Onyx, we recommend farming mining nodes in areas where you can mine higher value gemstones like Amethysts and Diamonds. Onyx itself is not as valuable and only sells for 300 coins. You can find these in The Forgotten Lands and the Frosted Heights. You should also try to mine while hanging out with a Disney friend who boosts mining. This can be any Disney character in your valley who you've set to boost mining after reaching friendship level two with them.
Looking for more help with Gemstones in Disney Dreamlight Valley? Check out Where to get Sapphires in Disney Dreamlight Valley, Where do Rubies spawn in Disney Dreamlight Valley?, or Disney Dreamlight Valley: How to Get All Gems here at Pro Game Guides.