Coalesced Radiance

When you cast this spell, you condense the ambient light from your environment into concentrated radiant energy and contain it within the gemstone focus. This first casting stores an initial damage charge of 3d10 Radiant damage. For the remaining duration of the spell, you can use your action to collect more ambient light and store it in the gem, increasing the charge damage by 1d10 for each turn you take this action. The spell ends if you use your action to do anything else. You can choose to end the spell early (no action required) at any point on your turn.

Because this spell condenses light into radiant energy, you must be standing in an area of bright light to cast or charge the spell.

When the spell ends, roll 1d6. The radiant energy remains safely stored within the gem for a number of your turns equal to the number rolled. While the energy remains safely contained within the gem, you or any creature holding the gemstone can use their action to release the stored radiant energy as a beam of searing light that is 90ft long and 5ft wide. Any creature caught is this beam must make a Dexterity saving throw against your spell save DC, taking the total charge damage on a fail or half as much on a success. If the energy contained in the gem is released in this manner, the gem remains intact and can be used again.

If, by the end of your turn a number of rounds equal to the rolled d6 after the spell ended, the energy within the gem has not been released or if you or another creature expends their action to crack and throw the gem up to 60ft, the radiant energy contained within the gem escapes catastrophically. The radiant energy contained within explodes in a 30ft radius sphere centered on the gem. Any creature caught in the blast must make a Constitution saving throw against your spell save DC, taking damage equal to the total charge damage and becoming blinded for one minute on a fail or taking half as much of the total charge damage  and not becoming blinded on a success. If the energy within the gem is released in this manner, the gem is destroyed.

At higher levels.

When you cast this spell using a spell slot of 4th level or higher, the initial charge damage is increased by 1d10 for each level above 3rd.

When you cast this spell using a spell slot of 6th level or higher, the amplification damage increases by 1d10 to 2d10 per round charged. The amplification damage increases by an additional 1d10 to 3d10 per round charged when cast at 9th level.