Explosive gems

[phiwum]

The explosive gem, like (for instance) the Lock/Knock spell, seems to promote use at the extreme end. For Lock/Knock, if you're locking something, there's little reason not to use five locks (provided you have the time to safely rest and restore the fatigue). For the explosive gem, there's no reason not to put as much ST into it as you can. If there are enough apprentices in your area, this would be an 8d gem.

Apprentices cost $25 per week. If we don't require a full week's pay, but only $5 for an afternoon's Aid plus $5 for the ST used, then a wizard might spend $40 for the apprentices and $50 for the gem to make a 6d explosive gem. That's a good one-use attack for $90. Add $20 to max it out at 8d.

I was thinking that there should be something more than an extra cost of $10 per 1d damage. Now, to be fair, finding six apprentices is not a triviality in my game, so that puts a cap on how deadly a gem can be. But I was also thinking of increasing the cost of the gem required to hold the charge. I thought I might start it exponential and then switch to linear, something like:

1d: $25
2d: $50
3d: $100
4d: $200
5d: $400
6d: $600
7d: $800
8d: $1000

Perhaps this is a bit too expensive. Just finding a $1000 gem may be quite a chore, though an 8d attack is pretty darned large. A petard costs $2500. It's not easily comparable, since its 6d radius is 3MH and it does substantial damage to 6MH. I just mention it as the nearest comparable. But the average damage of 28 points for an 8d gem is nothing to scoff at.

Any thoughts?