The lack of any advanced metallurgy among the Aztecs and Mayas has long been a mystery to students of pre-Columbian civilizations. Why, historians ask, were the great Mexican empires stuck in the Stone Age?

The Spanish crushed the Aztec empire with amazing ease, and the Americans’ technological inferiority was undoubtedly partly responsible. The conquistadors had gunpowder and horses; the Aztecs had neither. However, the blades of Aztec swords, made of obsidian, were sharper than steel. They could behead a horse.

Obsidian is a kind of glass formed during volcanic eruptions, and it shares all the basic physical properties of ordinary glass. The fact that the edge of a newly chipped flake is sharper than surgical steel was only discovered in the 1970s, and it has led to the use of obsidian blades in eye surgery, since the evenness of their cut permits much faster healing. The Aztecs called their obsidian-edged sword macuahuitl . Usually the swords were lined with ten blades; five on each side. Because obsidian is glass, it naturally fractures into a sharp, even, predictably shaped blade when chipped. Also because it is glass, it is brittle and cannot be resharpened. The blades on a sword undoubtedly had to be replaced after only a few uses; this is the main reason steel eventually supplanted obsidian after the Spanish conquest.

Why didn’t the Aztecs fare better against the Spanish with such effective swords? Probably because the swords didn’t have tips and were not meant to pierce; they were designed only for slashing. An adept Spanish swordsman could fend off an Indian simply by ducking a swing of the sword and then running the enemy through.

Obsidian was used extensively among pre-Columbian societies throughout North and South America, but it reached its highest development in the hands of the Aztecs. They used the blades for both hunting and warfare. Obsidian provided the projectile points for spears and arrows. Obsidian tools were used to shape the shafts of spears, arrows, and swords. Obsidian knives cut feathers and the cotton thread out of which mantles bestowed on successful warriors were made. The glass was used in butchering and in sacrifice. It was even used to cut a stillborn child to pieces before removal from the womb. And the versatility of obsidian made it a prized trade item.

The extreme sharpness of an obsidian blade may help to explain why the Aztecs were willing to submit to selfsacrifice. They cut their tongues and ears with obsidian blades on ritual occasions, caught the falling blood on the index finger, and flipped the blood in the direction of the sun or moon. Such self-mutilation was formerly believed to have been quite painful, but it is now understood that a fresh obsidian blade has an edge so sharp one barely feels a cut into the flesh.

In fact, the very sharpness of the obsidian blade is probably the primary reason the Aztecs had no metallurgy. To develop a metal that could do the things obsidian already did so exceedingly well would have taken too long. After the conquest, the Spanish adopted obsidian for many of their own tools, including shaving blades. It was not until the late 170Os that steel completely replaced obsidian in Mexican technology.

Another reason the Aztecs didn’t look beyond obsidian was its sheer abundance. More than fifty ancient sources of the material have been identified in Mexico and Guatemala, and they are the densest deposits in the world. The Aztecs usually drew obsidian from one mine at a time until the source was exhausted; then they moved on to the next. They surveyed for obsidian and other essential rocks by watching the early morning mists rising from the earth and searching for stones where the mists were densest. If and why this worked is not clear; perhaps the ground was cooler and retained less moisture where there were mineral deposits, and so condensation would be greater.

Obsidian is a semiprecious stone that can be polished, which also contributed to its unchallenged importance. The Aztec aristocracy wore obsidian jewelry; priests and nobles used highly polished obsidian mirrors to divine the future. In their animistic culture, everything, including rocks, had a spirit, and obsidian was especially revered. The principle god influencing life on earth, Tezcatlipoca, had an obsidian mirror in place of one foot. An obsidian knife placed in water in the courtyard of one’s home was believed to keep away sorcerers by frightening them with their own reflections. An obsidian blade was tied around the neck of a pregnant woman to prevent her child from being born harelipped.

For the Aztecs obsidian was not only almost ideally suited to all its practical uses, it also had roles in society that transcended the purely practical. It is almost impossible to imagine such a civilization forsaking this magical, useful, and abundant glass in favor of the methodical development of common metals.