Block of the Week: Obsidian

A mysterious material forged in the belly of the earth

Strong almost beyond compare and able to stand huge blasts, glossy, black obsidian is forged in the very fires of the earth. What a block! But real obsidian is a little different to the powerhouse you know in Minecraft.

As reflected in the game, obsidian's made when lava rich in silicates very rapidly cools, such as when it hits water. But while real obsidian looks like it’s also a deep, opaque black, if you hold it to the light you’ll usually see it shine with a green glow. That’s because obsidian is glass, and rather than being super tough, it’s brittle, shattering easily.

But this lends obsidian its greatest strength, something early humans knew all about. We’ve been using it for 700,000 years or so to make cutting blades and arrowheads, because broken obsidian has amazingly sharp edges. Carefully knap it and you can create a great variety of cutting tools, leading our Stone Age ancestors to establish obsidian factories and trade networks that could extend for a thousand miles.

Obsidian forms sharp edges because of its chemical structure. It’s incredibly hard, and unlike crystalline minerals, which tend to cleave along their structural lines, obsidian has no crystalline structure. When it’s struck, the shockwave freely radiates outwards from the point of contact, creating curved ripples across the new surface. The edge can be so fine that it rivals even diamond. At best, a razor blade’s edge is about 300 angstroms across (30 nanometers or 0.00003mm), while a good obsidian edge is about 30 angstroms.

This means even today obsidian remains a valuable cutting tool, excellent for very fine procedures such as eye surgery. Creating clean cuts without tearing, obsidian wounds heal quickly and with less scarring than with a steel scalpel. The only problem is that the blade is so brittle, easily shattering if hit from the side. That’s why it’s not used so much as much more expensive diamond blades.

In Minecraft you might build blast-resistant walls with redoubtable obsidian, but outside the game it’s a far more fragile substance. But it’s no less useful.