David Barnett
"When using diamond stones (fixed diamond plates) to shape or hone steel, do you use any kind of cutting fluid? ... It strikes me this should make a difference in the longevity of a stone."

I generally do not use lubricants on my diamond stones, Wiley, unless I'm doing a lot of work on one blade or sharpening several at once, and then, only to periodically wash away the swarf, the characteristics of which are well-described by Larry in his post, although I think his description better applies to the coarser grades of diamond than I use to sharpen steel, and more so for monocrystalline than polycrystalline diamond. Most of the time I just wait until I notice enough build-up to bother rinsing it away with tap water and maybe a drop of detergent or a finger-tip full of cleansing powder or a non-woven pad.

I don't think lubricant really does much to impart greater stone longevity, especially after it's well broken in. In a too-soft bonding resin or plating, diamond can be dislodged, pulled loose by the tug of either ferrous or nonferrous metals, although this is far more an issue with powered rotating wheels and discs than static stones. Some of the earliest stones did display this tendency and some of the current shoddier imports still do.

Lubricant is used on lapping with loose diamond grit or paste on cast iron and other laps, of course, but that's not part of your question.

Lubrication on diamond stones may enhance cutting action on new stones and uniform monocrystalline diamond filmsOn well broken-in stones I really don't see the need. It isn't absolutely imperative to lubricate diamond films, although it's usually best to do so.

A few other issues (about things you didn't ask, but I don't feel like starting another post and it's all running together in my head, so...) ...

I strongly feel it's necessary to differentiate between resin matrix and nickel plating, as Larry kind of lumps them together, as he lumps together powered diamond systems with stationary, as well. Resin-bonded wheels and discs and sintered diamond wheels/discs are designed to be soft enough to wear in the way Larry describes, to continually expose fresh diamond. Nickel plating, however, is designed to wear only at first, below the topmost layer, but once the stone is broken in, it nickel matrix should remain relatively immune to further wear. This is so for both nickel plated static stones and rotating discs; not so much for wheels.

Larry is also dead right that wheels and discs were originally designed and intended to cut harder, more granular materials than tool steels, and for the most part, still are, although there are exceptions. Even so, the exceptional uses, such as grinding, honing and polishing gravers on rotating discs, while still faster, more convenient and cheaper than not using diamond discs, can cause more rapid wear than if they were used on harder materials -- especially with clumsy or heavy-handed technique.

Interestingly, the accepted technique for grinding and sharpening gravers, whether HSS or specially-formulated graver steels, is to sharpen with no lubricant, no water drip.

Another point, when Larry says:

"... a granular swarf which is what the diamond mounting system was designed for.", ...

... I would agree this is so for powered wheels/discs and not so for static diamond stones. Even though nickel plating is employed on wheels, discs (resin-bonded, sintered ceramic-bonded, and nickel plated) and static sharpening stones, the latter are intended for tool steel use.

I do think Larry has taken what's true for powered wheels, discs, and burrs intended for cutting hard materials other than tool steel and applied it less correctly to diamond sharpening stones; the kind that sit still on one's bench.

Furthermore, when Larry says:

"Stones cut ferrous metals the same way, they make little shavings. Some of those shavings go around the diamonds but some are directed into the soft matrix. This undermines the diamonds quickly, much faster than when working material that generates a granular swarf which is what the diamond mounting system was designed for.", ...

... I would contend this is not correct for diamond stones. Undermining can be a problem with powered rotating diamond wheels and discs when applied to some steels, but less so to others, whether used with lubricants or not. Look at Bill's photograph again; see any egregious undermining?

As a longtime user of diamond for cutting and polishing gem material, stone, glass and metals, failure to draw or admit these distinctions only leads to confusion and inaccurate conclusions. It's easy to assume that some phenomena are bidirectional when they are not, and in this case, they are not. What is true for coarse diamond on powered systems does not make it so for fine (600-grit and up) diamond stones.

The simple fact that many of us have used diamond to sharpen and maintain tool edges and have done so for years without having to constantly or frequently or in many cases, ever upgrading or replacing our equipment is refutation enough.

Lastly (and back to your original question, Wiley), while I do not personally use lubrication on my diamond stones, there's little harm in doing so, a spritz or squirt with water (soapy or plain), WD40, light olive (subsequent pressings -- not EVOO) or mineral oil, glycerine-based lube/extender or whatever one prefers is fine. If your experience is that it is enhancing the cutting or extending stone life, by all means do so. I do things differently now than before, when I first started using diamond in its several forms, and have settled into what works best for me, or what I think works best for me.

Further lastly, I'm sorry if any, most, or all of the above is confusing, garbled, or otherwise incoherent. I'm having vision issues and can't proof, revise or effectively edit what I've written.