A few rulings came up in my DF game the other day, directly related to GURPS rules.
Spell Stones see a lot of use in my games, so questions come up all of the time. Here is how they play out in my game.
One Per Hand - you can't crush more than one spellstone per hand. You can't even hold two and crush one. You don't actually "crush" the stone. It's not fragile. You squeeze and concentrate (it takes a Step and Concentrate maneuver). It's not a question of physical force, but of finishing a ritual.
It takes deliberate action. It's not a free action, you can't "trade" an attack for it, and falling on a stone doesn't cause you to crush it against yourself. Unintelligent creatures aren't going to step on stones and set them off.
It casts a spell on the subject. In other words, the effect isn't set, the spell and energy is set. A 4-point Blur spellstone is worth -4 to hit on a SM+0 or smaller user, -2 on SM+1, -1 on SM+2 or +3, and worthless on larger subjects.
One exception is the Gem of Healing. It just works on anyone, providing 8 HP of healing when crushed. These are common in my game, but clerical spellstones are unusual as Spell Stone is an enchantment, and mostly wizards do enchanting. Ignore SM for these. It's an exception, and while I don't like exceptions, it's vastly easier to have it work universally than to scale it to SM.