This is an important question. In terms of green building it is my most important question actually.
I don't have any specific numbers and, honestly, getting accurate numbers would be quite a task. There are so many variables.
Broadly speaking, salvaged is always better than new. If something is salvaged, you are removing it from the waste stream. Garbage build-up is a big problem. You are also avoiding using energy to make something new.
No matter how green the new product it, no matter how much recycled content or good Karma points the sales rep brags about, a product that is salvaged always has less embodied energy. You could even argue that a salvaged product has zero embodied energy.
There is a huge industry busy selling green products, but at the end of the day the vast majority of them want you to consume more. This will unfailingly result in more material and energy consumption. Universal energy is limitless but Amazonian rain forest is not.
But the moment you salvage something you are saying, "Let me take that piece of garbage out of the dump and store it at my house instead." You are doing everyone a service and you are helping to lessen the burden that our consumer society has put on the planet.
So in terms of counters, if you salvage a piece of marble mined in Brazil, manufactured in China, and sold in the USA, I think it has less embodied energy than any CaesarStone.
But finding that salvaged marble is not always possible. So what do you do?
Salvaged wood is easier to get. And, in my opinion, a counter made out of salvaged wood is the greenest option. Even if it only lasts 20 years, it is greener than ageless stone.
After that, you have the recycled counters and concrete. I don't have numbers, so I'm only using what little I do know. The companies that use recycled products actually consume a lot of energy to heat their counters in order to make them strong. Concrete has its own problems. It creates huge amounts of CO2.
I find the best compromise is to make concrete counters on the site using your own recycled content. Glass or crushed stone, for example. It is not ideal but still better in my mind than buying CeasarStone or IceStone.
For more information:
GreenHomeGuide's Know-How article "Choose the Best Countertop Material for Your Home and the Environment" provides an overview of the lifecycle costs of popular green countertop products, from raw material extraction, to manufacture and transport, to end-of-life disposal or recycling.