Opalescence considered Rarity her oldest, most loyal friend and misstres.
Therefore, when she found out her time with her misstres and friend is soon to be over, she feels there is some things she need to do before it's too late, and something she needs to say.
Being unable to write all of this herself, she asks Fluttershy to help her write one last letter to Rarity from her loyal companion.

Written for a competition in the Weekly contest group