With plastic picks, the thinner the pick the more flexible it is. 

Hard picks produce a brighter tone, delicate, controlled and well-shaped. 

Thinner picks produce heavier less controlled sound. 

However there is really a lot to the personal preference when it comes to choosing a Guitar Pick. 

It is a fact though that thinner plastic picks give less attack and they wear down quickly. 

Jazz players and Bass players tend to use more heavy and thicker picks as they give much more control when doing fast tremelo picking. 

Stone Guitar Picks have no flexibility and their "heaviness" is inherent. 

This makes it possible to choose the thickness one likes the best regardless of the need for a heavier, hard pick. 

Materials: 

• Plastic 

There are various types of plastic that Guitar Picks are made of. This list shows them in the order of their stiffness 

• Celluloid 

First plastic ever used, flammable. 

• Nylon 

Smooth and Slick surface, can be used for very thin picks, short life cycle 1- 2 months. 

• Tortex / Delrex 

Brand names for DuPont Delrin, easy to grip with sweaty fingers. 

• Acetal 
• Ultem 

Highest stiffness of all plastics 

• Lexan 

Glossy, hard, wears out fast. 

• Metal 

Stiff, produces brighter sounds than plastic but is hard on the strings and wears them out quickly. 

• Stone 

Harder than metal, and therfore can resonate the strings more completely. Does not flex at all. Can be worked to any desired shape as no mould is required to produce them. They take a little longer to get used to. 

• Wood

 Usually Lignum Vitae, a very rare hardwood. 2-3 mm thick.