I was bored.
I'm assuming that a chasmfiends gemheart is roughly the size of a basketball, which I don't find too unreasonable given the size of chasmfiends, and the way it's described makes it seem like they can be held in one hand or two.
So that gives chamsfiends gemhearts a volume of roughly 7,104 cubic cm. Multiplied by an emerald density of 2.75 g/cm^3 equals 18,257.28 g.
Now the mass of spheres is measured in mg with broams being 400 mg, so multiplying that by 1000 and dividing it by 400 we get that a gemheart is roughly equal to 48,840 emerald broams.
But it gets better.
Going by this WoB
One clearmark is roughly a dollar.
Now a clearmark is worth 5 diamond chips, while an emerald broam is worth 1000
So multiplying our 48,840 broams by 1000 and dividing it by 5 we have a chasmfiends gemheart as being worth approximatly 9,768,000 USD. Almost ten million dollars per gemheart.
No wonder the highprinces fought so viciously over these things, and it puts a whole new perspective on Sadeas handing over a gemheart to prove he isn't in it for the money, he just handed over 10 Million dollars!
With one of these falling every week or so the highprinces must have made billions of dollars during the war of vengence.