Though a sphere is usually perfect, stone spheres may not be. The reason for that is the difficulty in cutting or carving certain rock and mineral materials. A gemstone or rock is a conglomeration of minerals. Those minerals may have different hardness, making it difficult to cut and polish a perfect stone sphere.
Some simple materials can be cut into gemstone spheres by machine. A cube of semi precious gemstone material is cut and then the corners are cut off. The new shape is put into cups in a sphere making machine and the machine slowing grinds out a sphere shape. It takes extreme talent to cut a gemstone sphere by hand using grinding wheels!
Also because they are cut from natural materials, the rock spheres may have flaws or inclusions. Sometimes they have fracture lines or surface pits. A naturally healed fracture line is where the earth shifted during the formation of the material, the material cracked or broke and then new material (it may be new of the same rock or a completely different material) formed in the crack resealing it. This is a natural record of an earthquake!
Not all inclusions are considered flaws. Some add character to the stone sphere. Some are inherent to the material and are to be expected. Open vugs or pockets with natural crystals are very desirable in a collector sphere.
The word sphere comes form the Greek word "sphaira" which meant globe or ball.
The sphere is an ancient and universal symbol that represents unity, completeness, and infinity. It symbolizes both the earth and the whole universe.
I'm not sure when the first gemstone sphere was carved. Crystal balls were used in the middle ages.
Gemstone spheres are measured by their diameter. The diameter is the imaginary straight line that penetrates from one side of the stone sphere to the other. If you need the exact size please refer to the mm measurement as it is more accurate!