Most will tell you you don''t need more than 10x. JohnQ would tell you anything more than 10x is cheating LOL But if you have a VS stone and you can''t find your inclusions with a 10x you might want a 20x. Anything above a 10x is mostly for fun though.
The whiz of Oz - there''s a joke in there someplace
- is correct, and I would add that if you''re not experienced with a loupe you might find it quite difficult to see much of anything without proper lighting conditions.
Remember, the larger the magnification, the smaller the width and depth of focus, so 10x with a 20.5mm fully corrected triplet lens is the best place to start.
I endorse what Garry and Gary have said about a larger mm size for beginners. Here are a variety of reliable loupes from Kassoy at 10X in different sizes: http://www.kassoy.com/loupes01.htm