system
Given the following post, respond as if you're commenting on Reddit:
human
Why are so many materials opaque? What's special about transparent materials like glass that sets them apart?

Also, why do other EM frequencies outside of visible light go through so many materials so much more easily?
gpt
There are two requirements for transparency:

1. No absorption in the visible EM spectrum. Absorption in the visible spectrum is usually associated with electronic transitions in the material. Some of the most common materials we use like plants and metal happen to have absorption in this region but most organic compounds actually have none. They are opaque due to reason two.
2. None or very few interfaces. Most materials like paper or cloth are white because unlike glass they don't have one homogenous shape, but instead consist of countless little strands of material.  In this instance, the light gets scattered instead of passing through like through glass. Essentially it bounces around inside the material due to reflection and refraction. This gives it a white but intransparent appearance. Take a transparent crystal and crush it into powder and it stops being transparent. Naturally occurring materials have tons of little defects, rough surfaces, inclusions and so on that transparency would be extraordinarily rare.

Combining the two just doesn't happen a lot with natural materials, since being crushed into tiny bits or being a messy combination of several materials is kind of the default state of most matter in nature.

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edit: Minor corrections: Saying that transparency is rare is actually kinda wrong. Water is transparent since its shape is defined by its container and always forms a homogeneous transparent body (unless heavily contaminated). 

Regarding absorption: Metals (or conductors in general) are also a bit of a special case since they can also reflect light instead of absorbing it. No interactions with the visible EM spectrum would be more physically correct if a bit unspecific.