This item is an enchanted crystal (of various type (Diamond, Quarts, Jade, etc. depending upon its value.))  When used (as a bonus action), the gem allows the holder to automatically upcast any ONE spell he/she casts in the same turn to by 1-6 levels, after which the crystal is destroyed.  The maximum level the spell can be upcast is dependent upon its value:

One Level: Agate,  Azurite, Quarts (blue),   Hematite, Lapis Lazui,  Malachite

Two Levels: Bloodstone,  Moonstone,  Onyx,  Amber

Three Levels: Amethyst.  Jade,  Pearl (white)

Four Levels:  Garnet,  Black pearl,  Topaz (gold \yellow)

Five Levels: Emerald, Opal, Sapphire,

Six Levels:   Ruby, Emerald (clear),  Diamond

For each level the spell is upcast, the caster takes one level of Exhaustion. The caster can determine the amount of the upcast he/she uses up to the maximum level of the crystal (this influences the amount of exhaustion).

The crystal is destroyed and the magic dissipates no matter what level the spell is cast at.

Notes: sleeping with the crystal next to the heart for a FULL long rest