Onyx countertops are favored for their beauty due to the varied and warm-colored hues running through them. It's perfect as a showpiece for your home, especially when there is suitable lighting to highlight it. However, because it's made of calcite, it is prone to etching by various acids that are commonly encountered in most kitchens such as those from strong cleaners and tomatoes. You would therefore need to clean it more than harder materials.
Preventing Acid Etching
To prevent various acids from etching your onyx countertop, seal it with a sealant that is food safe. Apply it according to what is specified on the package and do another application when water does not form beads on the surface anymore.
Cleaning the Countertop
Use a mild cleanser and soft cloth when cleaning your onyx countertop. Find something that has “green” printed on the label since such products are generally safer for onyx. Check to see if it is stone safe as well since many cleansers have too much acid for onyx.
For spills that happen on the onyx countertop, blot them off immediately. Never wipe them since the potential damage of the spill can widen. Leaving a spill for longer periods will likewise cause damage to onyx especially if it is acidic.