From Wreel Life, the Rare Earth wiki
Emerald City is the home of the elite silicon entities the Albinid. The crown of Silicon Mountain - it extends above the surface of the sulphuric acid super-lake. The green color is derived from the silicon-based coral used in designing the organic palace. Tear-drop pods - exponentially increasing in size - spiral like a galaxy toward a majestic central structure that houses the Albinid royal family.
The Albinid evolved over millions of years in a symbiotic relationship with a green crystal coral. At the earliest part of their relationship - the organism that would eventually evolve into the Albinid - took shelter in the crystal coral. Eventually the ancestral Albinid began molding this coral to further suit their reproductive needs.