If the word "earth" refers to the planet, shouldn't it be capitalized?

What does "serve as [something]" mean?

If something "serves as [something]," it means that it's suitable for a particular use. It can also mean performing duties for an organisation or country. Ex: The table cloth serves as a protective surface from spilt drinks on the table. Ex: The sofa also serves as a bed when we have people over. => an additional use Ex: He served in the army for two years.

Can you tell me how to use "kill something on" in a sentence?

This is an example of wordplay. To "kill the lights" means to switch or turn the lights off, and the speaker has used it both literally and figuratively to state that Broadway will be closed for business until 2021. Ex: The show's about to start. Can you kill the lights? Ex: The theatre killed the lights in preparation for the movie.

If I want to mention comedy genre of movie, 'comedy filled film' would work?

Yes! You can say "something-filled film" to describe a genre of a film, such as "comedy-filled film." "Suspense-filled" and "comedy-filled" are compound adjectives used to describe something full of that type of content. You could also use it for movies, series, books, and podcasts! Ex: I enjoy a good drama-filled series. Ex: She likes watching action-filled movies. Ex: I'm reading a romance-filled book at the moment.

What does "likely to ever be found" mean?

"Likely to ever be found" in this context means these gems seem to be the largest of its kind ever discovered; no other tanzanite that has been found was as large as these pieces. It's oddly worded in this sentence because it is usually phrased to mean something is hard or difficult to find. You also may know this, but the word "likely" means a high chance of something occurring.

What does "dig one's moxie"? Is it a common expression?

"Moxie" is an informal expression meaning a person with a strong fighting spirit (someone who does not give up easily). The speaker uses this expression to say that she believes that not many people would like ("dig") her fighting spirit. Ex: She has moxie. Nothing stops her. Ex: I have moxie. Not everyone likes that.