Rosin, the ubiquitous accessory for any stringed-instrument player is actually a bit of a mystery to most musicians. Unlike fretted or plucked instruments, rosin is a key element in making your violin, viola, cello, or double bass ring out its rich sounds. However, before you can choose the best rosin for your bow, you need to understand what it is, few know how it’s made, how it works, and which types or grades are best for their instruments. Read through this article, you should have those questions answered. What is rosin and how it’s made? Rosin, which is also known as colophony, is a resin collected from one of 110 different types of pine tree throughout Europe, Asia, North America, and New Zealand. Carefully heating the fresh, raw resin–sometimes mixed with other ingredients, depending on the recipe–vaporizes the volatile liquid terpene components. The cooking process and any added ingredients will change the playing qualities and color of the final product. It is poured into a mold while still a liquid. After the rosin has set, a cloth, or another application device is attached to the rosin. Why is rosin needed? Bow hair is great to use for playing string instruments, except for one thing: by itself, it cannot create the friction needed to cause a string to vibrate and produce sound! An application of a sticky substance, like rosin, is necessary to create the friction needed to cause the string to vibrate. The friction against the string actually causes rosin to melt momentarily, sticking to the string and pulling it, which activates the string’s vibration. How to choose a rosin? When purchasing rosin, first sort out whether you’re looking for a student- or professional-grade product. Student-grade rosin is cheaper, often has a grittier sound, and produces more powder than the professional grades. For some players, such as fiddlers, this is a plus. But classical players may find that the higher-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. Next, decide between light, or amber, and dark rosin–sometimes also defined as summer (light) and winter (dark) rosin. Darker rosin is typically lighter and stickier. However, with too much grip you’ll hear gritty sounds, since it won’t allow the bow to move smoothly over the strings, also dark rosin is usually too sticky for hot and humid weather—it is better suited to cool, dry climates. Lighter rosin is harder and not as sticky as its darker counterpart, it is also preferable for the higher strings. Lighter rosins tend to be harder and more dense—a good fit for violin and viola. Darker, softer rosins are generally preferred by the lower strings. However, also remember there there are different rosins for different stringed instruments. There are certain rosins created specifically for bass, cello, viola, and violin. Some companies also add precious metals to their recipes—another choice to consider when shopping for rosin. It is not uncommon to see gold, silver, lead-silver, and copper added to rosin. These materials purportedly increase the rosin’s static friction, creating different tonal qualities. Some companies also add precious metals to their recipes—another choice to consider when shopping for rosin. It is not uncommon to see gold, silver, lead-silver, and copper added to rosin. These materials purportedly increase the rosin’s static friction, creating different tonal qualities. However, there’s no one way to pick the best rosin for your bow right off the bat. Take your time and keep trying different brands until you find a perfect match. The right rosin for your bow and strings will make all the difference to your sound projection and playability.   reference: http://stringsmagazine.com/the-differences-between-dark-and-amber-rosin/