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Alec’s something old was a necklace that was a family heirloom, given that on their wedding day.
The necklace was from Jace’s grandmother, as a welcoming into the family.
Jace’s something old was a stone that got turned into a hair clip, the stone was blue like his eye.
The stone was found from Magnus in one he found in Peru, he and Alec paid for someone to turn it into a hairclip.
Magnus’s something old was a pair of cufflinks given to him from Alec’s father.
They received their something old as a good luck charm on their wedding, and now those items are being burned with their bodies, a good luck charm in the afterlife.