Fair question, and one worth answering carefully rather than with marketing.
Short version: yes, for normal play. But "balanced" means different things to different people, and how a die is made matters more than what it's made of.
What "balanced" actually means
A perfectly balanced die has its center of mass exactly at its geometric center. Every face has an equal chance.
No die achieves this. Not your $2 plastic set, not a $60 machined metal set, not a casino die. Manufacturing tolerances exist. Number pips remove material unevenly. Injection molding leaves voids.
The real question isn't "is it perfect." It's "is the imbalance large enough to notice in play."
For almost every die you've ever rolled, the answer is no. And measuring it requires either a saltwater float test or rolling the thing several hundred times and running statistics — not something that shows up across a campaign.
What affects balance in a fossil die
Density variation. Fossilized bone isn't uniform. It has porosity, mineral variation, and occasional voids — that's what makes each piece look different. It also means the material isn't perfectly consistent from one side of a die to the other.
How the fossil is distributed. This is the big one, and it depends entirely on construction method.
Natural flaws. Fossil has cracks. Some need filling. Filler has a different density than the surrounding material.
Three constructions, three answers
Pulverized and cast. The fossil is ground fine, mixed evenly into liquid resin, and poured as a single mass. Because the material is homogeneous before it cures, this is the most consistent of the three — there's no dense chunk sitting off-center, because there are no chunks.
Carved solid. Cut directly from a piece of fossil bone. This is the most "real" in the sense people usually mean, and it's the least predictable — you're working with whatever the fossil gives you. Small cracks and voids sometimes need filling, which introduces slight density differences. Still fine for play, but it takes more work to get there.
Suspended inclusion. A visible fossil fragment set inside clear resin. This one takes the most care, because the fragment has to sit at the center of mass or the die will favor a face. Done right, the fragment's weight isn't wildly different from the resin around it, and the difference stays imperceptible.
Here's the honest part: roll a suspended-inclusion die a thousand times and you'd eventually see it favor a face. In a normal game, across a normal night, you won't notice. If you're the kind of person who'd be bothered by that, we'd rather you knew now.
Should you care?
Depends what you're doing.
Playing D&D on a Tuesday? No. The imbalance in any well-made fossil die is smaller than the variance you'd get from your rolling technique, the surface, or the fact that you keep bouncing them off the Monster Manual.
Playing something with real stakes? Use plastic. Not because fossil dice are bad, but because "unusual material" and "certified fair" are different product categories, and anything with a natural material in it will lose that argument.
Collecting? Balance is close to irrelevant. You're buying a 66-million-year-old object you can hold.
The short version
Fossil dice are balanced well enough for normal play. Pulverized cast sets are the most consistent, carved solid takes the most work, and suspended inclusions require the most care to get right. None of them are casino-grade — and neither is the plastic set in your bag.