Locally quarried limestone may be the better environmental choice. The ingredients in concrete (aggregate, sand, cement raw materials) are also extracted from quarries, leading to the same environmental problems?including erosion and the leaching of minerals into streams. On top of this, manufacturing cement requires additional energy. Provided that the transport distance remains the same, I recommend you stay with limestone tiles.
Your question raises another environmental issue: the pool. In Southern California and other dry regions, this is very important. There are three primary environmental issues surrounding the use of swimming pools: water scarcity, energy use to heat the water and pump it, and the use of toxic sanitizers.
Water is becoming increasingly scarce in most western states, so the use of water for pools and landscaping is problematic from an environmental (and social) standpoint. Unfortunately, the fix is expensive. Ideally, you?d try to persuade your client to fill in the pool with dirt and replace it with a smaller water feature or lap pool.
If that?s not an option, consider other strategies to lessen the pool?s environmental impact. You could warm the pool with a solar thermal water heating system, and you could set up a photovoltaic system for running the water pump. A rainwater capturing and filtering system could augment the water supply, but only if your local building code allows it.
One easy and inexpensive step is to use a pool cover to minimize water evaporation and lower the cost of heating your pool. The Department of Energy's Consumer Guide to Swimming Pool Covers is a great place to start. According to the DOE, a pool cover can have the added benefit of reducing your pool's chemical consumption by 35 to 60 percent. When it comes to chemical sanitizers, this type of reduction may be the best approach. There are many alternatives to chlorine that claim to be eco-friendly, but they are more expensive, not as effective, and sometimes just as toxic.
For more information:
To learn more about the environmental aspects of different types of stone and tile, see GreenHomeGuide's Stone and Tile Know-How section.
For more energy-saving tips, check out the swimming pool heating resources on the Department of Energy's website.