Basalt

Basalt is an aphanitic volcanic rock that is incredibly common on Earth (90% of volcanic rock is Basalt.) Adding it as a new loose resource would open up the following mechanisms:

- Modern CO2 captures use Basalt caverns to inject CO2 which rapidly converts to Limestone. A late game CO2 capture enhancement could be a recipe to inject CO2 into Basalt in large quantities slowly with water and receive Limestone in exchange.
- Basalt is common in construction. It is used for groundwork, building blocks, cobblestones, statues, railroad ballast, and road base/asphalt aggregate. It could open up a later game Concrete recipe that is more efficient or be used in various construction projects especially the late game statues.
- Basalt could be spun into Stone Wool, which is a great thermal insulator, this could be a fun new consumer good your people demand to insulated their houses, possibly decreasing power usage of your city.