Not all items on the boulevard are made of the same material - some buildings are built out of glass or plastic polymers, while richer establishments are custom-grown crystal forms. What appears to be crystal cobblestones on the road are actually just decorative carvings on twenty-meter deep blocks of pure synthetic diamond. The foundation of the boulevard is five square hundred-meter blocks laid in a row, each twenty meters deep, made of diamond.
For eighty meters below that, it's layers of plates of latticed crystal-carbonate nanofiber armor. This is in fact a protective screen for a set of security bunkers for the three main branches of government and federation military command. This passive defensive system is effectively impervious to everything but antimatter warheads, and even then you'd need to practically level the entire city to get down to the bunkers.
The idea for this extreme protection came from the ultra-nationalist government that came into control of the Federation prior to the Gallente-Caldari War. The suggestion was made to allow for civilian structures to be built on top of the diamond shield, creating an exclusive and extravagant business area. When the Caldari admiral Yakiya Tovil-Toba deliberately crashed his ship onto Gallente Prime to buy his people time to evacuate Caldari Prime, it became clear that the ultra-nationalist idea for a super-fortified bunker might not be a bad idea. The bunker system and the protective shield was built, and the government opened up for civilians to rent space on the surface of the shield. The massive project paid for itself within 20 years.