Rosin.

Rosin is a solid resin that is commonly used by musicians who play string instruments. Its primary purpose is to provide friction between the bow and the strings, allowing the musician to produce a clear, controlled sound.

Rosin is derived from the sap of pine trees, though it can also come from other coniferous trees. The sap is harvested, purified, and then heated to remove excess moisture, resulting in a solid resin that is typically amber in color, though it can vary from light yellow to dark brown.

There are different types of rosin, and the choice of which one to use often depends on personal preference, the climate, and the type of instrument being played. 

Hard rosins are more suited for warmer climates, as they tend to remain less sticky in higher temperatures. Soft rosins, on the other hand, are more common in cooler climates because they are softer and provide a better grip in cold weather.

Dark rosin is typically preferred by cellists and bassists because it provides more grip and is better for heavier bows and larger strings. Violinists and violists often prefer light rosin, which is smoother and more suited to the thinner strings of their instruments.

Some rosins are mixed with various additives like oils or other resins to alter their consistency, grip, and sound production.

In Tamsta music store you can find a great choice of rosins.

https://tamsta.com/lt/460-kanifolija

https://www.pirastro.com/public_pirastro/pages/en/Rosin/