Seira's Compact is a box that holds Seira's pearl inside it until she is born (then she will keep it).
Lucia kept Seira's pearl inside her own locket (in the beginning of Season 2) until Sara gave her the compact. In the manga, Lucia keeps Seira's pearl next to her own so she can share her own pearl's energy with the orange pearl.
In Episode 57, the Black Beauty Sisters state that an orange light was coming from Lucia's locket.
Sara then gives Lucia the compact for collecting Seira's heart fragments in Episode 59.
In the later episodes of Season 2 it is unsure who is currently keeping the compact in their possession because Seira sacrificed herself but it appeared back in Lucia's hands.