Plastic thumbpick with shell print creates a warm, tonal response and increased control. Wear similar to a ring, but around the bottom of the nailbed. 
 
Thumbpicks can be used with almost any stringed instrument. Many fingerpicking musicians enjoy them because you can play two styles at once with ease, and it gives clarity and attack, especially to the bottom end. 
 
While picks come in different sizes to better fit your thumb, they also come in different thicknesses and materials (plastic and metal). The type of thumbpick you choose largely depends on your experience level and style of music.