Backlit onyx works because onyx is translucent — light passes through the stone instead of being reflected by it.
The standard build-up is: structural wall, an LED light sheet or a grid of LED strips, a translucent backing panel, then a 16–18mm polished onyx panel. The light must be even or you will see hot spots.
Book-matching matters most here. Two adjacent panels should mirror each other so the veins form a continuous pattern.
Use a dimmable driver and warm white LEDs (3000K or lower) — pure white washes out the colour of the stone.