Viola strings.

Viola strings are an essential part of the viola, impacting its sound, playability, and tuning stability. They come in different types, materials and gauges, and each choice influences the viola's tone quality and response. 

Steel Strings. These strings are made from solid steel or a steel core wrapped with other metals like nickel, chrome or tungsten. Steel strings are durable, stable in tuning, and less affected by changes in humidity and temperature. They are often used by beginners or for specific styles (folk or electric instruments). They produce a brighter, more direct tone compared to other string types.

Synthetic-Core Strings. These strings have a composite core made from various materials such as nylon or a mixture of fibers. They offer a warmer sound than steel strings, closer to the tonal quality of gut strings. They also provide better tuning stability and are less affected by humidity than natural gut strings. These strings are a popular choice for professional players and those seeking a balance between warmth and stability.

Gut Strings. Traditionally, viola strings were made from the intestines of sheep, but today gut strings are typically made from synthetic materials that mimic the properties of natural gut. These strings produce a rich, warm, and complex tone, favored by many professional and period instrument players. However, they tend to be less stable in tuning, more sensitive to temperature and humidity changes, and can be more expensive.

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

https://tamsta.com/lt/454-stygos-altui

https://www.pirastro.com/public_pirastro/pages/en/Stringfinder-00002/