Mill balls and “lonsdaleite diamonds”

As I note elsewhere, meteorites are not spherical. Nevertheless, I receive many photographs of spherical objects that people think might be meteorites. Here I discuss two special cases on spherical meteorwrongs – mill balls and objects sold on the internet as “lonsdaleite diamonds.”

Mill or grinding balls

A common way of grinding and pulverizing solid material is with a ball mill. The mill is a hollow cylinder filled with the material to be pulverized and spherical balls. The cylinder is rotated, which cause the balls to tumble and reduce the grain size of the solid material by impact abrasion. Mill balls are made of a variety of materials such as steel, tungsten carbide, glass, plastic, aluminum oxide (alumina) ceramic, and zirconium oxide or silicate.

Photos of mill balls that I found on manufacturers web sites. Note that not all are perfectly spherical.

A number of persons have sent me photos of spheres, often found in rivers, that are likely discarded mill balls.


Lonsdaleite is an allotrope of carbon with a hexagonal crystal structure (unit cell). It is sometimes called “hexagonal diamond” but this moniker is misleading and technically erroneous in that diamonds have a cubic crystal structure. (Analogy: If it has a hexagonal crystal structure, then it is not table salt because sodium chloride has a cubic crystal structure.) Lonsdaleite was first found in meteorites where it occurs only as microscopic crystals, usually less than a micrometer (0.00004 inches) in size.

A number of persons have told me that they “see” lonsdaleite in their rock, so the rock must be a meteorite. There are two problems with that logic. (1) If you can see a crystal without a microscope, then the crystal is not lonsdaleite. (2) Lonsdaleite can only be identified by determining that it is composed of carbon and proving that it has a hexagonal unit cell. That would require, at least, identification by x-ray diffraction. It can’t be done by eyeball.

“Lonsdaleite diamond”

At this writing, one can do internet searches for “lonsdaleite diamond” or “lonsdaleite hexagonal diamond” and find alumina mill balls being sold for prices ranging from $50 to $195,000 apiece. OK, the high priced one was actually advertised as a “Lonsdaleite Meterorite [sic] Diamond” and it was rather large at 2.5 inches in diameter.

A fellow identifying himself as a “researcher in particle physics” (but later “astrophysics”) sent me photos in 2023 of a marble-sized white sphere that he said had been found in Ohio. He identified it as a “meteorite of the name Lonsdaleite Diamond.” I politely told him that it was not a meteorite but that it was a man-made ceramic sphere. He replied “This celestial object is lonsdaleite. I’m sorry for you, your judgments are clearly missed. It is not a ceramic sphere, it has a lot of carbon which exceeds the natural diamond. Revise your words.” 

No. I’ll say it again:

If it is spherical and white, then it is not a meteorite, it is not a diamond, and it is not lonsdaleite.