A sphere is the most wasteful shape you can cut from a stone, which is why it costs more per gram than almost any other form. An obelisk — a four or six-sided tapered point — wastes far less, which is why the same stone is cheaper as a tower than as a ball.
Why size costs so much
Volume goes up with the cube of the diameter. An 80mm sphere is not twice the stone of a 40mm one — it is eight times. And it must be cut from a single flawless block that is bigger again, because everything outside the sphere becomes waste.
So the price jump between sizes is steep and it is not a markup. It is geometry.
Internal fractures are normal, mostly
Natural quartz almost always contains internal features: veils, feathers, rainbow fractures. In a sphere they are magnified and lit from every angle, so you see far more of them than you would in a tumbled stone of the same material. This is not a defect and it is often the most attractive thing about the piece — rainbow fractures throw colour when the light catches.
What to avoid is a fracture that reaches the surface, because that is a weak point and a sphere is a thing that rolls off tables.
It will roll. Get a stand.
Obvious, and routinely forgotten. A sphere needs a ring stand, a three-pronged stand or a recess. Buying an 80mm sphere and then discovering it will not stay on a shelf is a common and avoidable annoyance. Check whether a stand is included; often it is not.
Obelisks and points
An obelisk is a tapered column with a pointed tip, cut with four or six faces. Because it uses the length of the rough efficiently, you get a much bigger object for the money than a sphere of comparable stone. They also stand on their own.
Check the tip. Points chip, and a chipped obelisk tip is the first thing the eye goes to.
Care
Do not leave amethyst, rose quartz, fluorite or citrine in direct sun. All of them fade — amethyst noticeably so over months in a south-facing window. A sphere sitting in sunlight is also a lens, which is a genuine fire risk on a wooden sill and not a theoretical one.