Screening your material (sand and gravel) before panning is is crucial! Screened material is much easier to pan as you remove larger materials. Screening is especially important when it comes to panning tiny grains of gold, so called micro gold.
The net itself is made from stainless steel.
This is a simple chart explaining how to convert mesh to mm.
30 mesh = 0,6 mm
50 mesh = 0,3 mm
100 mesh = 0,15 mm