Frank Kuzma is releasing a new arm!

Here is what I found about the stiffness of Sapphire, albeit on a website for a company that sells things made of sapphire for industrial use:

"Sapphire is one of the hardest materials on earth. Mechanical hardness is typically measured using the Mohs scale: glass generally has a Mohs score between 6 and 7, and hardened steel which comes in at 8 Mohs. Sapphire has a Mohs hardness value of 9, placing it just below diamond which has a hardness of 10 Mohs.

Furthermore, sapphire is very stiff. Its Young’s modulus is 435 GPa, making it 6 times stiffer than quartz, so it can’t be stretched or deformed easily.

These properties make sapphire one of the strongest and most durable materials on the planet."

Frank Kuzma is no dummy, I totally agree.  I greatly admire his turntables, and based on the kudos accorded to them, I also admire his tonearms, until now, possibly. But sometimes even smart guys over reach.  We shall see. You wrote,  "no, you can't entirely divorce inertia from mass but it does matter where the mass is."  But in fact the equation for "effective mass" (not mass alone) already takes into account the distribution of mass in a tonearm, as I am sure you realize. Ergo "effective mass" is closely related to inertia. Perhaps Kuzma ameliorates the problem by using a variable damper counterweight, a la the Technics EPA100.

I also was wondering before this discussion ever came up, after having read Fremer's reveal of the sapphire tonearm, is sapphire really so much stiffer than other stiff but not so dense materials of which tonearms are conventionally made? For example, the Technics EPA100 arm wand, titanium nitride. And the Mk2 version which was made of boron something. Or even steel?

Let’s spare ourselves the obvious jokes about its cost. The elephant in the room is that this tonearm is an elephant. At 60g effective mass, not many if any cartridges will qualify, if you base tonearm matching upon the equation for resonant frequency or even bring it into the buying decision? And yet its mass will be ignored by the well healed cognoscenti, because it is so expensive. In the minds of many with big bucks, cost can overcome the constraints of the physical universe. Even Fremer, who mentioned this product, its cost and its effective mass, in his most recent newsletter, didn’t bat an eyelash over that last specification. We shall see how it is ultimately viewed.

Just read Mijostyn’s last post after having posted this. I don’t see how you can divorce effective mass from inertia.