Amulet of 3 elements

This amulet is set with 6 gems: 2 rubies, 2 sapphires, 2 topazes.
Any gem pried from the amulet crumbles to dust. When all the gems are removed or destroyed, the amulet loses its magic.

You can use an action to cast one of the following spells (save DC 15), using one of the amulet’s gems of the specified type as a component:
Ruby (Fireball) 
Each creature in a 20 foot radius Sphere centered on that point must make a Dexterity saving throw.
A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.
Sapphire (Ice ball)
Each creature in a 20 foot radius Sphere centered on that point must make a Dexterity saving throw. 
A target takes 8d6 cold damage on a failed save, or half as much damage on a successful one.
Topaz (Lightning ball)
Each creature in a 20 foot radius Sphere centered on that point must make a Dexterity saving throw. 
A target takes 8d6 lightning damage on a failed save, or half as much damage on a successful one.

Notes: By a spell caster