Aluminum oxynitride — ALON, marketed as "transparent aluminum" — is a real, manufactured ceramic. The U.S. military uses it as bulletproof window armor: a 1.6-inch ALON pane stops a .50 BMG round that punches straight through 3.7 inches of laminated glass. So what happens if we stop using it for gun ports and start using it as the entire structural envelope of a 500-meter tower?

The material, by the numbers:

• Density: 3.69 g/cm³ (concrete: 2.4; steel: 7.85)
• Compressive strength: ~2,900 MPa (concrete: ~30 MPa)
• Flexural strength: ~700 MPa (structural steel yield: ~345 MPa)
• Young's modulus: 334 GPa (steel: 200)
• Fracture toughness K_IC: ~2.4 MPa·m^½ (steel: 50+)

That last number is the catch. ALON is roughly 100× stronger than concrete in compression and stiffer than steel — but its fracture toughness is twenty times worse than mild steel. It's a glass-grade ceramic. Tap it with a hammer in the wrong place and a crack will sprint across it at ~6 km/s.

Sizing the columns. Take a Burj-class tower at ~500,000 tonnes. The base must carry ~5 × 10⁹ N. With concrete at 30 MPa working stress (and a safety factor of 4), you need ~670 m² of column cross-section — which is why the Burj's basement looks like a forest of fat pillars. With ALON at a comparable safety factor on its 2,900 MPa compressive strength, you need just ~7 m². The entire vertical load of a megatall could pass through four columns the diameter of a dinner table.

The building would be visually almost gone. Light transmission through 2-inch ALON exceeds 80% across the visible band. Stack thirty floors of it and you're still looking at a luminous crystal lattice, not a wall.

Why we shouldn't. Three problems, in increasing severity:

1. Brittleness under dynamic load. Wind gusts, seismic acceleration, and bird strikes all dump energy into a structure as transient stress. Steel absorbs this by yielding plastically. ALON cannot yield — it fractures. A 1% overstress event that would leave a steel beam slightly bent leaves an ALON beam in pieces. You'd have to over-design by perhaps 10× and pre-stress every member in compression, since ALON's tensile strength is half its compressive.

2. Thermal shock. Coefficient of thermal expansion is ~5.8 × 10⁻⁶/K. A 40 °C swing across a 100-meter column gives 23 mm of differential length change. Constrained, that's a stress of ~78 MPa — survivable, but a fire on one face creates gradients that will spall the column like dropped Pyrex.

3. Cost. ALON sells for roughly $10–15 per square inch at 2 inches thick — about $300,000 per cubic meter. The Burj Khalifa contains 330,000 m³ of concrete at roughly $150/m³ (~$50M). Substituting ALON volume-for-volume: ~$100 billion, or about 1.5× the GDP of Luxembourg, for the structural material alone. Manufacturing is the bottleneck: ALON is hot isostatically pressed in vacuum furnaces, and the world's total annual output is measured in tonnes, not kilotonnes.

Scale the production fifty-thousand-fold, solve the brittleness with a glass-laminate sandwich (two ALON skins around a ductile interlayer, like architectural laminated glass on steroids), and you get a building that weighs the same as a steel tower but disappears into the sky.