Cassandra De Vries' Necklace is a key, found in DataDyne Central: Defection which can open the secret passage to the underground laboratory elevator. If Cassandra dies, the key will not function and the mission fails.
Another copy of the Necklace is dropped by Cassandra in the mission Attack Ship: Covert Assault on Perfect Agent difficulty. It is a replacement for the one Joanna took back in the first mission, but has no utility in this mission, as the username and password shown when looking at it in the pause menu was probably originally intended to be used in some way, but this was never implemented by Rare.
Cassandra De Vries' necklace. Fitted with a proximity key tag to allow access to restricted areas. MK3 model only functions while owner is alive. Security class AA-I (gamma).
Cassandra De Vries' replacement necklace. Username: CDV780322 Password: I8MOZYM8NDI8S.
By replacing the 8s in the password above with As, it spells "I AM OZYMANDIAS", which could be a reference to any of a number of characters or works.