I'm toying with the idea of creating an unstable magic item that, once per day, allows casters to cast a spell at a higher level than normal with the potential for a mishap. I'm not sure if the effect is too powerful for a common item or if a 1/6 chance is too common for a mishap and would make my players not want to use it. Any advice is appreciated.

Here's the description for the item as it stands

While holding this crystal, you may expend a bonus action to empower the next spell slot you use by one level, allowing the caster to cast a spell using a lower spell slot than usual, or allowing the caster to increase the power of a spell that could already be cast using the chosen spell slot. Additionally, if the spell deals fire, cold, acid, poison, lightning, radiant, or necrotic damage, then you can treat any 1 on a damage die as a 2.

The first time you cast a leveled spell after using this crystal, roll a d6. On a roll of 2-6, the spell is empowered as intended. On a roll of 1, roll a d20 and refer to the mishap table below.

Once you use the crystal it may not be used again until the next dawn. Using more than one novacrystal between long rests increases the chance of a mishap. The second novacrystal used will provoke a mishap on a d6 roll of 1-3, any novacrystals used beyond the second will cause a mishap on a d6 roll of 1-5.

Mishap Table

• 20: The spell is empowered as intended.
• 12-19: The spell succeeds but is not empowered. If the spell slot used is insufficient for the spell cast, the spell fails but the spell slot is not consumed.
• 7-11: The spell fails but the spell slot is not consumed.
• 4-6: The spell fails, consuming the spell slot. Roll on the sorcery wild magic table to determine a secondary effect.
• 3: The spell succeeds and is empowered but is cast at a random target in range.
• 2: You must make a saving throw using your spellcasting ability (DC15). You take 1d6 psychic damage per level of the spell slot used on a failed save, or half as much on a successful one. The spell slot is consumed.
• 1: All creatures within 5 feet of the crystal, must make a dexterity saving throw (DC15). Creatures take 1d8 piercing damage per level of the spell slot used on a failed save, or half as much on a successful one. The spell slot is consumed and the crystal is permanently destroyed as a result.

Yes, that is very, very powerful for a Common item, it should likely be Rare or Very Rare.

Also, if I may make some small suggestions:

Novacrystal

Wondrous Item (crystal), rare (requires attunement by a Spellcaster)

This magic crystal has 1 charge. While holding it, you can use a bonus action to expend this charge, potentially empower the next spell you cast using a spell slot. The next time you cast a spell of 1st-level or higher within the next [time frame], roll a d6. On a roll of 2-6 the spell is empowered. On a 1 the spell surges, roll a d20 and refer to the surge table below.

An empowered spell has the following benefits:

• It is 1 level higher that the spell slot used to cast it.
• If the spell deals acid, cold, fire, lightning, necrotic, poison, or radiant, thunder damage, treat any result of 1 on a damage die as a 2.

The crystal regains it’s expended charge daily at dawn.

Using more than one novacrystal in a single day increases the chance of a surge. The second novacrystal used will cause a surge on a result of 1-3 on the d6, additional novacrystals surge on a  result of 1-5 on the d6.

Surge Table

• 20: The spell is empowered as intended.
• 18-19: The spell is cast as normal.
• 15-17: The spell is cast and consumes a spell slot one level higher than intended. If a spell slot of the required level is not available, no spell is cast, and no spell slots are expended.
• 12-14: The spell is not cast, and no spell slot is is expended.
• 7-11: The spell fails, consuming the spell slot.
• 4-6: The spell fails, consuming the spell slot. Roll on the sorcery wild magic table to determine a secondary effect.
• 3: The spell succeeds and is empowered but is cast at a random target in range.
• 2: The spell fails, and consumes the spell slot. Make a DC 15 saving throw using your Spellcasting ability. You take 1d6 psychic damage per level of the spell slot on a failed save, or half as much on a successful one.
• 1: The spell fails, the spell slot is consumed, and the crystal explodes. Each creatures within 5 feet of the crystal must make a DC 15 Dexterity saving throw. A creature takes 1d8 piercing damage per level of the spell slot used on a failed save, or half as much on a successful one. This damage counts as magical for the purpose of overcoming damage resistance and immunity. The crystal is permanently destroyed.

Wow, this is more than I could have possibly hoped for. Thank you so much.

I will definitely take your changes into consideration. I don't have much experience with power scaling so balancing this item has been gnawing away at me for a while now.

Again, thank you.