Why do I need rosin?
Because rosin is sticky. When you apply rosin to the hairs of your bow and draw the bow across the strings of your violin (or viola, cello or bass) the rosin grips the string and tugs at it. The bow keeps moving and the string snaps back to its original position and is caught again by the rosin on the hair and the cycle is repeated. This happens very, very quickly. In the case of your A-string 440 times per second. If a bow’s hair has never been rosined it will not grip and the hair just slides over the string and you hear nothing.
What is Rosin?
Rosin is a sap collected from many different types (over 100 different types) of pine tree throughout the world. The color of rosin is influenced by the time of year it is collected. If the resin is tapped in late winter or early spring, it will be gold or amber in color and hard when set up. As the seasons change to summer and fall, the color of the resin darkens and the consistency softens.

Light or Dark?
Choice of rosin is a very personal thing and everyone seems to have their own favorites. The purpose of rosin is to provide grip to the bow hair. You can think of rosin as sticky or smooth, hard or soft or dark or light. Dark is softer and stickier. You might choose dark rosin in winter when the air gets dry.

During the summer when the air is more humid you might choose light. But many players find one they like and never change. Players of larger instruments often prefer dark rosin. How much should I spend? Student-grade rosin is cheaper and has a grittier sound. It produces more powder than the professional grades. For some players, such as fiddlers, this is a plus. Classical players often find that the high-priced professional-grade rosins better fit their needs. Professional-grade rosin is created from a purer resin and generally produces a smoother, more controlled tone.

What else is in my rosin?
Some companies add precious metals to their recipes. It is not uncommon to see gold, silver, lead-silver, and copper added to rosin. These materials create different tonal qualities. Gold is warm; silver bright.