You're all welcome to assume you have a Murder Jewel of average quality. If you want one of above average quality, you'll have to cough up 6000 GP. Jewels above that quality aren't admissible at chargen.
I'm trying to hash out the exact benefits for now, but the general idea goes like this:
Each Jewel has a "grade quality". These usually range from very poor to excellent usually (or exceptional, which are much, much rarer).
Jewels don't offer any passive benefits of their own. What they do is enable you to power up your abilities, according to A) the strength/grade of the Jewel, B) the knowledge stored in the Jewel, if any, C) the amount of time you've already spent channeling power through the jewel and D) the amount of power you're trying to extract from the Jewel itself.
According to their grade and strength, each jewel has a certain amount of "safe time" in rounds per encounter that they can be used. This number is typically equal to your Base Will Save Bonus OR your Base Fortitude Save Bonus, whichever is lower. (Higher grade jewels add or multiply that number.) As a general rule, using more than half those rounds (rounded down) in one encounter will diminish the number of "safe" rounds in the next by one.
Whenever you activate a Jewel, you may choose to try and boost a class feature by a factor of X (where X is dependant on the grade/strength of each Jewel). So for spellcasting, you can boost CL, for Rage, you can boost duration, etc.. Said factor can be increased at the cost of extra "safe" rounds. So, if you choose to boost your power by X+1, each round the "safe" rounds diminish by two instead of one, and so on and forth (the maximum factor is determined by jewel grade, and you are never allowed to know the exact number. You will never know the exact number of "safe" rounds either.
)
So what happens if you run out of "safe" rounds?
You go berserk.
Typical effects include attempting to boost the power to the limit against your will (not as in will save; you take a special level check with a bonus equal to level plus the highest of your mental stat mods), going frenzied berserker on your buddies,
confusion, pain, etc.. The exact effect is almost never the same.
If you try to boost your power beyond the jewel's ability, the jewel shatters, and you become consumed with uncontrollable madness. The exact effects of this also vary from person to person. So no two will be alike.
One can also make use of multiple Jewels to diminish individual power drain and make them less likely to shatter. This is typically dangerous because the exact effects when a Jewel DOES shatter in this instance are... not exactly known.
Sealing one's powers inside the jewel enables that someone to detect active magic users or creatures by instinct. However, if separated, the victim then becomes a crazy mess. If the Jewel is shattered after separation, well...
Nothing much happens when someone who sealed their powers inside a Jewel shatters it through overuse, however (not beyond going berserk anyway).
One can also seal one's memories inside the jewel. This causes disorientation, pain, and temporary drain to your mental stats. It does, however, allow someone to share in said memories later. (It may or may not hold more permanent effects. But I'll keep you guys wondering.)