Symbolism, Imagery, Allegory
From the very first page of Splendors and Glooms, it's clear that the phoenix-stone is quite the formidable object—one that both bestows a great deal of magical power on its owner and eventually curses him or her to a fiery death. That's the problem that Cassandra finds herself facing: She covets and holds the stone close because it's the source of all her power, but she also fears what will happen to her if she hangs onto it for too long:
It was the stone within the locket that burned her. She kept it caged inside the gold filigree: a fire opal the size of a crow's egg, blood red, veined with ribbons of changing color. For seventy years she had cherished it. Now it fed upon her, burning her and sapping her strength. (P.6)
But, the phoenix-stone isn't impervious. Even though Cassandra cannot destroy it (because she wants its power too much), it can be destroyed—as long as someone truly doesn't care about the power they can get from it. In the end, Clara is the one who ends up defeating the evil, cursed stone because she's not interested in it for her own power or greatness; she just wants to make sure her friends don't get hurt by it. She wants it gone because it's going to destroy Parsefall's life if he ends up taking it:
You need only wish for the stone, and you will be yourself again. If your wish is strong enough, your strings will snap and Grisini's spell will be broken. If Clara wished, she might save Parsefall; she could save him if she stole the stone herself. The curse would fall upon her. Clara shut her eyes in terror. (46.4)
At the end of the day, the phoenix-stone is something that only brings misery, and because of that, it has to be destroyed by someone who is acting out of goodness and love. Good, it seems, triumphs over evil in this book.