Essence is at all points inadequately 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 nearly massless.
That, and the fact that essence can be recharged merely by standing within a certain radius of an Essence Pool, imply that essence is gaseous and is taken in through the breath.
This theory is widely accepted but unconfirmed, and therefore non-canon.
Non-Shaper NPCs regularly cast attack spells that require essence, implying that they can and must also work with the substance; however, non-Shaper laboratories or residences never contain Essence Pools.
On the few occasions where essence is manufactured non-organically, the equipment and energy consumption are both massive.