If all your ski mountaineering takes place entirely on snow, then you don't need a diamond stone.
However, if your skiing surfaces include occasional interludes of rocks and other non-snow surfaces, this diamond stone is an absolute must for your tuning kit.
Or even if you don't have a tuning kit, this diamond stone will still be hugely helpful all by itself in removing overhanging burrs caused by close encounters of the rock kind.
If you do have a tuning kit, or at least have the ability to sharpen your edges with a metal file, you'll still want to use a diamond stone first to spare excessive wear on your metal file from attempting to remove burrs, plus rock hits can often case harden a metal edge: if your file is suddenly skipping over a section of the metal edge without sharpening it, a few passes back and forth with the diamond stone will magically remove that case hardening effect.
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