Simply put, the water ring you see is either a stain or an etch... or a combination thereof.
You must first determine if the “stain” is a true stain or an etch mark. A general rule of thumb when dealing with stains on natural stone is that a stain will always be darker than the stone. This means the stone has absorbed contaminants such as oil, grease, dirt, etc. An etch, on the other hand, will always be lighter than the stone. Etching occurs when an acidic substance comes in contact with a calcite based stone such as marble, travertine or limestone.
With potted plants that sit directly on natural stone, there are a couple of possibilities. If the ring you see is darker than the stone, then soiling has penetrated into the pores of the stone and left a stain. Moisture that accumulated under the pot may have contained acidic properties that could have reacted with the stone surface causing the stone to etch.
We first suggest cleaning the stone surface thoroughly with a heavy duty stone cleaner to clean deep down into the stone to remove any ground in dirt and soil. (Ask us for recommendations.) Rinse thoroughly, then let dry. If the ring is still there, then it's time to address the stain or the etch or a combination of both.
Removing a stain (remember, a stain is always darker than the stone) will require the use of a poultice. A poultice is an easy and effective way to draw stains out of your stone. A poultice is essentially a cleaning/chemical agent to break down the stain and an absorbent material to draw it out. There are many ready-made poultices on the market today, but you can easily make one yourself. (http://stoneandtilepros.com/stain-removal-application).
If the stone happens to have any etching (damage that is lighter than the stone), it could be possible to restore this yourself. Minor etching that is smooth to the touch can possibly be removed with a marble polishing powder or compound IF (and this is important) the stone is light colored and polished. If the stone is honed, or if the etch mark is rough to the touch you will need to contact us to restore the surface of the stone.
TIP: Avoid placing potted plants directly on the stone surface. Condensation or moisture from spillage could result in a stain or etch.