Marble, travertine, limestone & onyx are all soft materials which are prone to marking as a consequence of their calcium carbonate structure. Acidic liquids react with calcium carbonate and literally eat away a tiny piece of the polished/honed surface, creating dull spots which are typically called an etch. Contact from acidic materials like lemon juice, wine, coffee, fruit juice etc can create an etch. The more acidic the liquid the faster and deeper the etching occurs.
An etch is not a stain; like a scratch it effectively changes the surface of the stone. It generally appears as a light, dull mark. Penetrating Sealers cannot stop etching as they are beneath the stones surface and not a physical barrier between the acidic liquid and the stone.
Calcium based stones have three main factors which relate to maintenance, they are porous they etch and scratch easily. Although a penetrating sealer cannot stop etching and scratching, it can reduce the stones porosity which will provide stain resistance.
How to Repair an Etch?
If the etching is minimal (you can see it, but is not rough enough to feel) you can polish it out using a marble polishing creme such as Aqua Mix® Reviver®. If the etch is deep and you can feel the surface is rough, you will need to have it mechanically removed.