The G2[b]Z[/b] has a Pyrex (toughened glass) lens.
The G2 [entry level model] has a Lexan (polycarbonate) lens.
Both are coated with a variety of anti-reflective and scratch resist layers.
The G2Z Pyrex bezel has the "Hex" anti-roll style where as the G2 Lexan bezel has 12 rounded bumps that go some way to reducint rolling.
The G2Z and G2 bezels can be swapped round. Indeed people buy the G2Z bezel (by calling SureFire) so that their G2's can use the P61 High Output Lamp Assembly without fear of melting the Lexan lens of the standard G2 bezel.
In general, SureFires are not intended to be used "constant-on" rather for shorts bursts and flashes. If used constant-on, models such as the 9P, 8X, 9N etc with outputs of over 100 Lumens and small 30-35 mm diameter bezels do run the risk of melting their Lexan lenses.
With the output of the P61, there is a chance that the G2's Lexan lens will be damanged for prolonged constant-on use. Nitrolon does not remove the heat from the bezel like Aluminium does. Therefore models such as the 6P, D2/D3, Z2/Z3 and C2-BK/C3-BK with their Lexan lenses are at risk also. A black anodised bezel with a Pyrex lens can also be bought by calling SureFire.
If anyone would like to see photos of the G2 and G2Z together, or the M2 with other SureFires to get an idea of scale, please email me.
Al