system
Given the following post, respond as if you're commenting on Reddit:
human
Why is a lump of coal black, but a diamond is clear?
gpt
Coal is a sedimentary rock composed of many minerals. Diamond is a mineral composed of a carbon-based crystal structure. If impurities are present in that structure, the clarity of diamond (or any mineral) can be altered. That is why we see varying colors of diamonds.

A better question would ask why graphite (also a cabon-based structure) is gray/black and diamonds are clearer. Both are considered allotropes of carbon with different crystal structures that propagate light differently, which varies their appearance. 

Fun fact: Diamond is not stable at our atmospheric temperature and pressure (think ground surface and areas we inhabit) and will eventually break down to graphite. Although, this could take a few billion years.