Re: the difference between crystal and fused quartz

SiO2, in all its forms (quartz and clear fused silica being two of several)
is made up of little tetrahedra consisting of a silicon atom surrounded by
4 oxygen atoms in a tetrahedral arrangement. In pure SiO2, each oxygen is 
shared between two tetrahedra...that is, the tetrahedra are essentially 
joined together at their tips (it is possible to come up with arrangements 
where the tetrahedra share two oxygens or three oxygens, but only if other
atoms are present). What makes the various forms of SiO2 all different is 
variation in the spatial arrangement of the tetrahedra. Quartz has a 
moderately complicated but regular and repeating pattern of tetrahedra.
Fused silica has tetrahedra in a semi-random arrangement. That is, in any
local area (over  the distance of a few tetrahedra), it is fairly regular, 
but there is no long term order.

Many books on crystallography have diagrams and illustrations (rather im
possible to recreate in text as I type here) that will help to clarify this 
for you. Try p. 33 of Sinkankis' "Mineralogy", which shows a number of the 
possible binding arrangements of the tetrahedra (this covers more than 
simple SiO2). Or try Bloss' "Crystallography and Crystal Chemistry". Bloss 
just happens to be the text I own, but therer are any number of good 
crystallograpy texts will have similar diagrams and discussions.

Silicates in general can be very fascinating. Almost the entire crust and
mantle of the Earth are made of them. The variety of forms and
composition of the silicate minerals is almost unending.