This hand-crafted mala's sterling silver pendant represents the vajra which symbolizes the ritual implement held in the lama's right hand and has been translated as thunderbolt but also diamond.  The Sanskrit word vajra has the connotation of adamantine, that is, "unbreakable," or, "impregnable."*   Therefore, besides being able to dent any object and overwhelm with its incomparable brilliance, the vajra or dorje represents great durability -- a hardness plus an immutability that is practically eternal. Ultimately, it stands for:

The mystery of the mind, the omniscience, the pure awareness of all the Buddhas,
Indicated by a symbol of eternal strength and constancy,
The vajra heart of knowledge and emptiness is like the sky --
How wonderful to see the intrinsic face of reality!

~ translation of Tibetan verse under Nyingmap