The butterfly knife was Beth Kane's primary weapon of choice. The garnet inside it was originally inside a necklace worn by Beth when she was young.
Beth Kane acquired a butterfly knife as she became the villainess "Alice." She implanted her birthstone, a garnet from a necklace onto to the knife. Alice would twirl the knife around in her hand and uses to kill her enemies.
In 2018, Alice uses the knife to kill some police officers who were present at the shut down of the Bat-Signal. Later, when her gang kidnaps her sister Kate, Alice holds the knife as she talks to Kate. Later, when again confronted by Kate who's are under the identity of "Batwoman," Alice throws her knife at her, but Kate dodges it. After Kate manages to save Sophie Moore, Alice disappears, but leaves her knife behind, prompting Kate to take it. As Kate examines the knife, she notices its ruby matches the one on her own necklace, causing Kate to suspect that Alice might her supposedly deceased sister.
Meanwhile, Alice who realizes she forgot her knife, sends her boyfriend Dodgson to retrieve it, but he does not find it as Kate already took it. Angry at the loss of her knife, Alice terrorizes an old couple living in her former childhood home and throws a tantrum.
As Kate tries to examine Alice's DNA from the knife, she and Sophie are attacked by a group of thugs hired by Kate's stepmother, Catherine Hamilton-Kane to steal the knife. Once the thugs successfully steal the knife, their leader presents it Catherine who orders him to destroy it.
It's unknown what happens to the knife afterwards, though it can be assumed it was successfully destroyed.