Ioneos

Ioneos is like Kuon's "older brother," as he and Kuon are created from the same type of magical crystal. However, Ioneos' appearance and size are much larger and sleeker than Kuon's because he is comprised of an entire crystal, save a small chipped fragment which Kuon is made from. He was sealed inside of one of the school's stone pillars until Hikari accidentally released him during a routine fire drill. The entire school was subsequently demolished after the blast following Ioneos' release. Jennifer nicknamed him "Oniita," saying that his original name was too long. In the end, Ioneos turns into mud along with Kuon and become crystals. Kuon being the little chip and Ioneos being the big crystal.

Jennifer puts them on a necklace and wears the crystal. from wikipedia