VPI 3-D tonearm

Is the VPI Prime 3-D tonearm considered high-mass?
I think it's more of a medium mass
They do not list the mass of that tonearm on the specifications. I have never seen that omitted on any arm I have owned. I wonder why? It is an important spec to mate it with the cartridge you want to use.
I agree.
I want to add a stone cart from Koetsu but don't know if it'll be a good match.
Harry has said that he's installed almost every cartridge into the 3D with success. If it were I, I would call VPI with your question, but having many arms, none is as much as a "no arm" than is the 3D
I have use a Koetsu Azule on a 12-inch 3D arm. It sounds amazing. Prior to that, I had to use Damping fluid in a 12-inch metal VPI arm to get it to sound right.
I have a 3D arm on my Aries 3 with an Ortofon Cadenza Black - mates very well.
I have a Lyra Titan-i on my 10" 3D tonearm- sounds great!
It is ESSENTIAL to know the effective mass for proper cartridge matching!

Why is VPI hiding this spec??? What are they afraid of???

I guess they do not believe in science, or think it will work optimally with "any" cartridge. LOL!

By the way, they have never listed the effective mass of any arm in the past.
The effective mass of the VPI 10.5" 3D tonearm is 10.6 grams
Harry told me that effective mass is an unimportant consideration with the 3D because it has nearly no resonance at all. That's what makes it sound so good.
With all due respect to Harry, while the resonance level of the 3D arm may be relatively low, every arm and cartridge combination will still have a resonant frequency. The total effective mass of a tonearm and cartridge will determine where that frequency lies. The frequency of that resonance is an important consideration when matching an arm to a cartridge. Ignoring it can result in combinations with the resonant frequency within the range of record warps, and that can never result in optimal performance. Of course, YMMV.