What is the best sharpening stone

Google search for "what is the best sharpening stone" produces more than  7 780 000 results.
Most of them is pure marketing, dealing with one or two parameters and following well known model of "Mind's Limit: 2 Things at Once".
But good sharpening stone has more than 2 parameters ! Not to mention the best. Here we review all of them, but not in details, if you need more technical information, read this article. 

Last but not least to mention, that this article targets  professional sharpeners and novices. If you need stone for personal use, in kitchen  - buy anything called "sharpening stone". Anything called "sharpening stone", made from any material harder than steel, will sharpen your knife, better or worse. But if you need  accurate angle and ideal surface roughness - knife sharpener with a set of sharpening stones - is the only choice. 

1. Material, or abrasive 

   Abrasive is what makes cutting edge sharp, so it should be harder than steel. There's no other parameters except hardness which makes sharpening stones productive. Obviously diamond, as the first hardest material in nature is the best for sharpening. But it also has disadvantages. Diamond  losses the hardness very rapidly with temperature rising above 700⁰C (1290⁰F). The second hardest material, cubic boron nitride (CBN, borazon, cubonite, elbor, etc.) for temperatures above 1000⁰C (1830⁰F)  performs better than diamond. Also, while  machining steel with diamond,  steel extracts carbon atoms from the diamond abrasive, and diamond grit erodes. 

   CBN abrasive, in contrast to diamond, contains no carbon atoms and this  makes CBN better suitable for machining of hardened steel and HSS. So, in high speed grinding diamond due to its hardness, is suitable for machining all carbides, but is not suitable for machining steel. 

   How it's relevant to  manual knife sharpening, considering you hardly reach such high temperatures ?  If you sharpen occasionally, diamond sharpening stone should last longer than CBN sharpening stone of the same parameters (dimensions, concentration, grain size), because it's harder, which means more productive. But I do not know if such test were made... 

     Answer on question, "which sharpening stone is better, diamond   or cubic boron nitride" is just  matter of habit and  discussions on your favorite knife sharpener's blog/forum. 
Yet, we believe, CBN stones with bond, designed for specific grain size is optimal choice. 

2. Grain size, grit, granulometry

Sharpening stone must ensure maximum sharpening quality and minimum surface roughness of the knife. Selection of abrasive grain sizes depends on actual and desired surface roughness. Coarse grain sizes are used in grinding, ultrafine grain sizes (below 0.5 micron) - in mirror polishing. How many grain sizes to be used between these operational steps, depends only on... your budget. Recommended set of sharpening stones consists of 4-5 grain sizes. 

Sometime   sharpening stones are marked as "coarse", "fine" or smth like that. Looks like selection was made for you... But is it professional to group 20 grain sizes into 3, like was called above ? I saw stones with grit 45 micron  called 'coarse" by known manufacturer.  Amateurs...  Product listings here contain Grain size/operation reference charts.
  
3. Bond

Sharpening stones' bonds are generally classified as:
- electroplated (galvanic), 
- organic (resin, rubber, and so forth),
- metal (copper-tin), with each type of bond offering its own unique benefits. 

   Electroplated bond is a single layer of abrasive particles held by a tough, durable nickel alloy. Word "single" should be enough to explain how long is working life of electroplated sharpening stones, although all we know electroplated diamond nail files perform very well while shaping nails. 

   Metal bonds (e.g., copper-tin) offer excellent wear resistance and form-holding ability of sharpening stone (dressing is required less frequently than for resin bond stones). Metal bond sharpening stones last longer, and have higher removal rate. 

   Resin bond (e.g., phenol-formaldehyde) is softer than metal bond  and is perfect, for example for such delicate operations, as mirror polishing (with grain size 0.25/0μ (60000 US MESH)). 

4. Concentration, amount of abrasive. 

The more abrasive is in a bond, the longer is working life of a tool.  Decreasing abrasive concentration is the simplest way to decrease cost of tool. If not indicated in documents, lower concentration is very difficult to recognize at the first sight. Concentrations below 100% is a waste of money. 

Conclusion

The best diamond sharpening stone or CBN sharpening stone is a product, which parameters conform to standards (grain size, bond, concentration) and are fully disclosed in specification.  This makes selection of sharpening stone weighted  decision, which is based on requirements of your working operation (productivity, desired and initial roughness). Two products are highlighted to underline that for professional sharpening 2 abrasives should be regarded: diamond and cubic boron nitride.