Bacillus pasteurii is a “microorganism, readily available in marshes and wetlands, [that] solidifies loose sand into sandstone.” Check out a BLDG BLOG post about a proposal for the Sahara desert here.
“The structure is made straight from the dunescape by flushing a particular bacteria through the loose sand… which causes a biological reaction whereby the sand turns into sandstone; the initial reactions are finished within 24 hours, though it would take about a week to saturate the sand enough to make the structure habitable. The project – a kind of bio-architectural test-landscape – would thus go from a balloon-like pneumatic structure filled with bacillus pasteurii, which would then be released into the sand and allowed to solidify the same into a permacultural architecture.”
The final result can also be imagined by observing Tafoni stone formations, even though in this case it’s erosion rather than aggregation that’s taking place. There’s some of this stone in Germany, actually.