Underpowered at level one, but progression is steep enough to make it OP by lvl 2; also, flat attack bonus has to go.
The main problem I see with your spell is not that it's not balanced, but that it's designed in a significantly different way from PHB spells, so that it doesn't work well within D&D 5e paradigm.
First of all, flat bonus to hit is bad. Why is it bad? Because D&D 5e introduced bounded accuracy, which reduced the spread AC could have. In particular, spells based on attack roll are good against two types of targets - low AC (because they are easy to hit even by wizards) and high AC, but also Legendary Resistance or high saves (i.e. those who are hard to hit, but also can just lolnope your save-based spells that hit automatically). Thus, having a flat attack bonus can increase the usability of a spell noticeably, even if it's just +1 - for example, Rod of Pact Keeper gives exactly that for Eldritch Blast, and it is a Rare magic item. And the kind of bonus your spell has... The highest AC a monster can have is only 25; your spell can hit Tarrasque if cast at level nine, even if it is cast by wizard with 4 Int who rolled anything higher than a nat 1! And on a reasonably built character, this spell basically autohits anything a character could encounter at spell level 5 or 6. That's the one of effects that makes Magic Missile have such low scaling damage. Speaking of which...
I get that you probably was trying to balance this with the need to have rocks available and placed so that can levitate unobstructed. But as written - one can get 100 pumice pebbles weighing 1g, spend a turn spreading them out on the floor and then deal full 100d4 damage, because damage is not tied to size or mass of the rock thrown. By the way, that's way too much for level one spell even if mediated by the need to hit (compare level 2 spell scorching ray: cast at level 9 it gives you 20d6 potential damage, i.e. both less min damage and max damage, requiring ten attack rolls even on single target), and as we previously established - the attack bonus basically means that it's hitting automatically. And that's not even accounting for the chance to crit. That happens, in part, because your damage progression is not linear at higher levels - again, something other spells don't do.
Since this is a ranged attack - it can easily be used to induce concentration checks (or death saves!). That's the second reason Magic Missile has such low per-level damage; and again, at higher levels flat attack bonus again makes this use much more reliable. Yes, you can't make several attacks on one target - but you still can throw your rocks on several spellcasters per turn.
Again, progression runs away fast. By level 4-5, you outrange all other attack spells. Moreover, long-range disadvantage is mitigated by flat attack bonus (do you see the pattern here?).
Your spell does not specify, which class it is designed for. If it is restricted to single class, it might affect balance - for example, if it is for a class that does not have much direct damage spells.
Let's look at other spells' material components. As you can see, unless the spell specifies that the component is expended on cast, it is usually something character found once and, bar plot twists that take away PCs' possessions, will have readily available always. It's very rare that it's something that players would rely on GM to provide, like berries for goodberry. You are trying to do something different here. It's not inherently bad, but it could get away from you - for example, if DM and player will have different opinions on how big of a room you need to fling full 100 rocks around or how fast a player can get a hundred spherical rocks rolling on the floor.
Lastly - usually spells have very simple progression. They take one, rarely two parameters and increase it for the same amount for each level; that keeps progression in check and spell description free of complex tables.
I advise you compare your spell to chromatic orb. This spell also deals damage (3d8, +1d8 per level) on attack roll @ 90ft range; you can choose the damage type, but the spell also requires an expensive gem as a component. It's probably the closest 1st level spell to what you are making. Trading off the gem component for damage types is probably balanced; if you add more features (like splitting attacks on multiple targets or increasing range or maybe giving the caster advantage on attack roll) - compensate by reducing damage.