While I totally agree with your sentiment, there's are a couple problems. First, there is a missing period. The comma should come after the "and", not before it, so the sentence can make sense on its own without the aside. I can't tell if they meant to voice this in past or present tense since they bounce back and forth. "Let" suggests past tense, while "lets" would suggest the diamond's growth is ongoing, which I think would suit it better. Ultimately, there are some poor choices in words and grammar that make this aphorism inherently more flawed than a diamond.