When Bruce Dahlin underwent lung surgery here in early December, the operation, while under the most modern conditions, contained an echo from the age of the ancient Aztecs.

At Dahlin's request, the surgeon made his first cut with a scalpel fashioned from obsidian, a rocky glass from volcanoes that the Aztecs used to make knives and razors.

Dahlin is the first human operated on as the result of an unusual research project that weds modern medicine with archeology and its study of the distant past.

Firmon E. Hardenbergh, an eye surgeon in Boulder, Colo., and Payson D. Sheets, an archeologist at the University of Colorado, have found that obsidian can be made at least 100 times sharper than most steel scalpels. Indeed, obsidian shatters to form edges only 10 molecules thick.

Now the two men are trying to determine whether the glass blades might safely and effectively replace steel for some surgical procedures.

Dahlin, an archeologist at Catholic University here, became intrigued by the research and asked Sheets to make obsidian blades for his surgeon to use for the first cut through his skin. He was impressed, in part, by reports that the sharper obsidian causes less scarring.

"The incision looks pretty good right now," Dahlin says.

The obsidian study is in its early stages, and Hardenbergh has reached no conclusions about the glass' usefulness in human surgery.

Nonetheless, he described the project to a gathering of eye surgeons in Houston last October and concluded: "Gentlemen, I submit we may be on the brink of a New Stone Age."

This month, Hardenbergh plans to begin using obsidian blades in a series of eye operations, primarily cataract procedures.

Obsidian is a relatively rare product of volcanic eruptions.

"It requires very specific conditions to form," Sheets says.

The glassy rock is found throughout parts of Mexico and Central America, where the Aztecs and Mayans of old used it for various cutting tools.

"The Spaniards preferred the obsidian to the steel razors they had," says Sheets.

The Mayans and Aztecs used obsidian blades for ritual bloodlettings. The Aztecs also used obsidian to cut out the hearts of human sacrifices. That gruesome act, Sheets notes, "demonstrates how effective obsidian is in cutting into the [chest] cavity."

Obsidian's apparent medical advantage lies in its sharpness. In a detailed study with an electron microscope that magnified the edges of scalpels 50,000 times, only one steel blade approached the keeness of obsidian. Ironically, that blade is no longer marketed because it proved too fragile for normal operating-room use.

The sharper a blade, the less damage to tissue. Hardenbergh explains, "The less cellular trauma there is, the less healing there has to be."

He is interested in obsidian's use in eye surgery because a keener blade allows cutting with less pressure. The eyeball is filled with fluid and when more pressure is used, more fluid escapes.

"It's going to come out, but the question is whether it is going to come out slowly or quickly," Hardenbergh says.

A rapid release of fluid can push out the eye's iris, which is difficult to put back in place. And the change of pressure within the eye caused by a quick escape may damage its blood vessels and cause hemorrhaging.

Sheets knows of one case in which a man, an amateur archeologist, had chest surgery performed with obsidian blades he made himself. The site of his incision was almost invisible.

If, indeed, obsidian blades cause little scarring -- a fact as yet unproven -- they could become important in plastic surgery and other operations where patients don't want scars showing.

A number of questions need answering. For example, how durable are obsidian blades? Will they dull quickly or break easily? Are there any unforeseen safety problems?

Such questions must be answered before obsidian blades find a place in the nation's operating suites.