Essence is only sparsely described, though it is consistently said to possess the "acrid scent of ozone," and is several times referred to as "clay-like." However, Shapers are said to be able to store essence inside themselves, implying that it is either nearly massless or that Shapers have some organ to contain it within themselves.
Essence is said to be mixed according to a "complicated and secret recipe," and also contain "proteins and other organic substances" that can break down or denature after a long period of storage. It also bubbles, fizzes, and steams, meaning that it is highly energized. It is possible that essence is a type of "primordial soup" charged with magic, a series of complex protein chains, amino acids, and other building blocks of life that have been enchanted for ease of use by Shapers, and allow them to "spontaneously" Shape life by magically rearranging this raw organic material into a new life form.
Non-Shaper NPCs regularly cast attack spells that require essence, implying that they may also work with the substance; however, non-Shaper laboratories or residences never contain Essence Pools. Shapers may use essence, with its energized nature and organic makeup, to reduce the energy expenditure needed for their own spellcasting; in this way, it may serve as a sort of liquid catalyst or battery.
On the few occasions where essence is manufactured non-organically, the equipment and energy consumption are both massive.