Explanation 1
---------------
Thinking in terms of frequency.
A very regular solid with few free election can be transparent because it has a narrow spectrum band
Less electrons --> Less energy level of electrons
Strong bonds --> Less free electrons --> Less energy level of electrons
Less energy level of electrons --> Less spectral lines
Strong bonds --> More regularity (bond are more exact length) --> More exact energy levels --> Narrower spectral lines
Less spectral lines + Narrower spectral lines -->narrow band spectrum absorption --> More transparent
SiO in Glass will be more ordered and thus more transparent with the characteristics above
For non transparency,
First consider a clear plastic cup that you then crush and reshape, you may notice that at the stressed regions, there is a more white translucent color. It is a known phenomena that an antenna with x+1 resonance, if placed close to another antenna with x resonance, both antenna resonances change slightly. It you keep adding more antenna, each resonance is affected by the group. and all may not be at the same level.
In the ordered solid plastic cup, each molecule is in exact placement. Now when the plastic cup is bent, the polymer order is more disrupted, and there is more variation in molecular distance, and changes in the energy levels. Each molecule behaves as an antenna, and each resonant frequency changes slightly. Further more the proximity to other antenna causes more variation. Now there is a broadband interaction with spectral lines and the cup stress regions looses it transparent more narrow band spectral
The same thing with stone, impurities and irregularities in the crystal formation at a bigger level causes a more broadband spectrum
Explanation 2
---------------
Diffraction in orders glass is basic. Visible light pass tru and at most if at an angle there is a rainbow effect but still transparent
In powdered form, micro crystals cause multiple diffractions, thus eventually and effectively reflecting back the light and also mixes back the separated rainbow colors that was reflected, in a random way but still give back a white color
Let's act on what we agree on now, and argue later on what we don't.
Black men leave Barbeque alone if Barbeque don't trouble you