To figure out who the pink bag belongs to, read the spell inside the bag ("Siren's Revenge") and pay special attention to the "Ingredients" section.

One of the ingredients for the spell is the birthstone of the person doing the spell. There is a stone in the bag.

Do you see stones referenced or depicted in any other pieces of evidence?

Look at the digital extraction of Luna's computer. Is there anything on it that you can connect to the stone in the bag?

The digital extraction of Luna's computer has a circular diagram that shows the symbols for the 12 astrological signs. There is a stone depicted for each astrological sign.

Which astrological sign does the stone in the bag correspond to, and who has that astrological sign?

The stone in the bag is purple. This corresponds to the Aquarius' birthstone (Amethyst). Aquariuses are born between January 20th to February 18th.

Whose birthday falls between these dates?

If you're having trouble figuring out the suspects' birthdays, check their witness statements.

The spell in the pink bag lists the birthstone of the person casting the spell as an ingredient. There is a purple stone in the bag. We can infer that the person who owns the pink bag has a purple birthstone.

On the digital extraction of Luna's computer, there is a diagram that depicts the birthstones for each astrological sign. The birthstone that looks most like the purple stone is the Aquarius' birthstone (an amethyst). Aquariuses are born between January 20th and February 18th. Based on the birth dates listed on the witness statements, Luna is the only Aquarius (born February 10th, 1986). The pink bag is Luna's.

The 4 relevant pieces of evidence are: Purple Stone, Spell, Digital Extraction of Luna Brooks' Computer, and Suspect's Witness Statement or Police Interview (Luna's witness statement).