2 Answers

Resin is a natural or synthetic compound which begins in a highly viscous state and hardens with treatment. Typically, resin is soluble in alcohol, but not in water. There are a number of different classes of resin, depending on exact chemical composition and potential uses. There are numerous applications for resins, ranging from art to polymer production, and many consumers interact with products which contain resin on a daily basis.

Natural resin comes from plants. A classic example is pine sap, which has the characteristic sharp odor of terpene compounds. As anyone who has interacted with pine sap knows, the substance is very viscous, but it hardens over time. A number of other plants produce resins, and plant resins have been used by humans for thousands of years. Some plants exude a similar substance called gum or gum resin which does interact with water. Gum tends to be softer and more malleable than resin.

That darkened stuff at the end of joint or dark hardened or gooey inside ones bowl/bong, also.