Sausalito, Calif.--Call it the secret life of coral.

Calera, the green cement company founded by Brent Constantz, has been embroiled in controversy for over a year. The company claims it can take carbon dioxide and seawater and produce calcium carbonate, or limestone, which can then be mixed into cement or other industrial products.

He ticks off the potential benefits: The world mines 36 billion tons of aggregate and limestone a year and generally, these materials are harvested from surface mining operations. Producing construction filler from CO2 would thus curb this environmental hazard. Likewise, it would take factory emissions that otherwise would contribute to global warming and transform them into an economically valuable commodity.

Critics, meanwhile, have maintained that the transformative process generates more carbon dioxide than it sequesters and consumes an inordinate amount of energy.

The critics, though, don't fully understand calcium carbonate, Constantz told me in a hallway interview at a conference sponsored by Khosla Ventures yesterday. (See more on Calera's business plan in the video.)

"Calcium carbonate is just a chemical formula," he said, denoting a substance that consists of molecules containing calcium, carbon and oxygen atoms in a specific ratio. The key is that different crystalline forms, or polymorphs, of calcium carbonate occur naturally. Think of carbon. Diamonds, carbon nanotubes, graphite and the dust in charcoal briquettes are all made from carbon: the difference lies in the structures formed by the carbon atoms.

Textbooks identify around nine forms -- Aragonite, Calcite, etc. -- of calcium carbonate, he said. If you studied Calera's business plans with those forms involved, you could come to the same conclusion to which the critics have jumped.

But in reality, approximately 30 polymorphs occur in nature. Many of the more exotic forms produced by corals, however, are initially unstable and revert to one of the more familiar nine forms listed in textbooks, he said. Calera manages to exploit these exotic polymorphs in the way it controls the reaction between seawater and carbon dioxide. It does not rely on magic catalysts.

"There is a whole suite of polymorphs we didn't know existed," he said. "We are copying the low energy pathways of mineralization of corals."

Not many people study the subject, but Constantz has been doing just that since his days as a Fulbright scholar. Bone generation and bio-mineralization have been the basis of his previous startups and sixty-something patents. Some of the insights into these polymorphs have come through his work at the Stanford Linear Accelerator.

Corals and ancient life forms eliminated the extraordinarily large amounts of atmospheric carbon dioxide on the planet 100 million years ago the same way. It's how we got the White Cliffs of Dover.

"99 percent of the CO2 on the planet is stored in limestone," he said. "The way the Earth has dealt with high CO2 levels is make calcium carbonate."

Is he right? It sounds intriguing but we shall see. In any event, don't expect the controversy to end. More debate and data will follow.

Meanwhile, we've got some other home movies of the Khosla Ventures companies. Here is CEO Donald Runkle on the modular efficient engine coming from EcoMotors. A full-fledged version of the engine -- which could run on diesel or gas -- will be tested in a car later this year. The picture shows a prototype.

EcoMotors will have to team with car companies to make it in the market, but auto makers aren't as prickly about dealing with startups as one might think, he told me. He ran GM's advanced technology unit. GM didn't pour $1 billion into the EV1 electric car as a publicity stunt. The company wanted it to work: the problem was that the contemporary technology couldn't add up to an affordable car.

Every automaker also licensed Wankel rotary engines. "Only Mazda could make it work" economically, he said. The company has already signed a deal with one Chinese manufacturer to explore commercialization.

Junkers, aircraft supplier to The Third Reich, experimented with opposed piston/opposed cylinder engines like the ones EcoMotors hopes to make, but the efficiencies never quite worked. These engines differ, among other design tweaks, in that two pistons facing each other occupy the same cylinder. Porsche has made opposed cylinder engines.

Finally, here is Tom Gielda with a component from an air conditioner from New Pax that runs on water and consumes 75 percent less electricity than normal units.

"We don't pump gas. We pump liquid," Gielda said.

Tags: air conditioning, cal star products, calcium carbonate, calera, carbon sequestration, cement, controversy, diesel, ecomotors, energy efficiency, engine, gas, green building, khosla ventures, moss landing