Killing Attack - Ranged 1d6+1, Variable Advantage (+1 Advantages; +2) (60 Active Points); OAF Fragile (Gems; -1 1/4), Limited Power Power loses about a fourth of its effectiveness (must have the right type of gem; -1/4)
Expendability of the gemstones is the relevant bit, I think. If the gemstones aren't expended, eventually he's going to wind up with the top quality one and use it all the time. This isn't worth a Limitation. He starts by buying the spell at the base level (quartz, for instance), and then spends XP on the full-power version when he gets a diamond. Yes, he could lose the diamond (and go back to the quartz version temporarily), but that shouldn't last more than a gaming session. (Playing out repeated attempted robberies would get boring for everyone.)
If the gemstone goes away permanently, things get more complicated. You buy the spell at the lowest level, with the lowest level of Expendable (-0). Don't worry about keeping track of it at this level, just assume that he has a decent supply of quartz chips or whatever bits the spell needs. Then, add additional points of effect that require a Difficult to replace focus (-1/4). The highest levels of effect require a Focus that is Very Difficult to replace. Here's how it might work:
Magic Blast: (Total: 40 Active Cost, 24 Real Cost) Blast 4d6 (20 Active Points); OIF Expendable (Easy to obtain new Focus; quartz chips; -1/2) (Real Cost: 13) plus +Blast 2d6 (10 Active Points); OIF Expendable (Difficult to obtain new Focus; zircon crystals; -3/4) (Real Cost: 6) plus +Blast 2d6 (10 Active Points); OIF Expendable (Very Difficult to obtain new Focus; cut diamonds; -1) (Real Cost: 5)
If all the caster has are the quartz chips. the blast is 4d6. If he has zircon crystals, it's 6d6, and if he has cut diamonds available, it's 8d6.
I would recommend a variable power pool for this situation.
Then he can set powers based on available gemstones and you and the player can work out an active power cap for powers based on gemstone quality. Maybe even power types based on gemstone type.
