Quintessence is 'found' in two forms, although only one of them can be touched. It flows through the pattern of everything in existence, essentially keeping them in existence. It can also often be found as Tass; Quintessence trapped inside a physical existence, and when loosed can be absorbed much like meditating at a Node, where the stuff is found.
The true nature of Tass is practically unknowable. Since Quintessence is immaterial and free-flowing, it shouldn't hypothetically possible for Tass to exist, yet it does. Metaphors of Quintessence being water and Tass being ice are the most common way of explaining the abnormality, but it goes nowhere towards explaining why something that should travel through a pattern could get trapped by it. Some theories are that excessive Quintessence at a Node has to go somewhere; Tass doesn't trap Quintessence. it merely has an extremely long pattern that can store a large ammount of Quintessence inside. Another theory is that Quintessence isn't getting used enough by the world; trapped by the static nature of the world engineered by the Technocracy, a lot of Quintessence simply has nowhere else to go and 'condenses' at Nodes.
For all intents and purposes, Tass is 'solidified' Quintessence.