To remove heat marks from Onyx, you can follow these steps:

1. Mix a small amount of baking soda and water together to create a paste.

2. Apply the paste to the heat mark on the Onyx.

3. Gently rub the paste into the heat mark with a soft cloth or sponge.

4. Rinse the area with warm water and dry with a soft cloth.

5. If the heat mark remains, repeat the process or try using a marble and granite cleaner.

Note: Do not use any abrasive cleaning agents or rough scrubbing pads on Onyx as they can scratch the surface.