Ruby Sharpening Stones

[jeffbird]

What are ruby sharpening stones used for?

Manufacturers say they are second in hardness to diamond plates.

Congress Tool, among others, offers them, so there must be some professional tool and die makers or machinists using them for some application.

 

[Bill DeShivs]

I use them to sharpen knives and engraving tools. The finest ones will leave a mirror finish.

 

[jeffbird]

I use them to sharpen knives and engraving tools. The finest ones will leave a mirror finish.

Bill,

are there certain steels or knives where you use the ruby stones over the others?

 

[Obsessed with Edges]

'Ruby' is aluminum oxide, of which its hardness will be essentially similar to any other aluminum oxide abrasive. They all rate around ~2100-2300 on the Knoop hardness scale, as compared to abrasives like silicon carbide (SiC) at around 2600-2800 Knoop, cubic boron nitride (CBN) at something over 4500 Knoop, and then diamond at 7000 Knoop as the reference for the hardest of all. I attached a comparative hardness chart from Norton below.

Other factors, like the toughness or friability (resistance to fracture or breaking down) of the abrasive grains and grain shape can make different forms of aluminum oxide either much more aggressive for heavy grinding, or somewhat gentler and more finessed where polishing is concerned.

 

[FortyTwoBlades]

Ruby is considered the toughest of the aluminum oxides. The higher the chromium content the darker the red.

 

[Bill DeShivs]

As I said, I use the ruby stones to sharpen/polish the tips of my gravers. They are used for engraving metal.
Some of the new knife steels are difficult to sharpen and require diamond stones or coarse AO stones like India stones to get an edge.
The sintered fine ruby stones will polish those edges easily.