Most commercial "glass" would be considered a borosilicate. As such it has, at least, boron in the mix. You have to pay "extra" for the purer "quartz-type glass" (only SiO2). It's one of the reasons that many mirrors used in lasers are so expensive. They are "quartz" as well as being ground exceptionally flat. In addition, commercial "glass" has many other additives to make it "do" what is desired (flow, float, skin, etc.)
Here's some pic's to show the difference. I cut larger sphere out of "fused quartz." It is basically "glass." I cut the smaller sphere from a quartz crystal. It is pure SiO2. I took two shots so you could see that it wasn't the light passing through one that made the second different. The beam is from a 150mW 405nm laser. Notice that the color of the beam passing through the real quartz is virtually unaffected by the sphere, while the color of the beam going through the "glass" is very different
I didn't ever notice this until I saw this thread . I experimented around the house and it works on every lager glass object, the trails are red-orange and few are yellow-green.
+ it makes a yellow trail on some of the white painted walls and the on fake wooden floor (w/e its called) too! Wow! [smiley=2vrolijk_08.gif]
. I experimented around the house and it works on every lager glass object, the trails are red-orange and few are yellow-green.