How is this achieved?

Ravensblack has the right idea.  Also, heed the warning and DO NOT BREATHE THE DUST!  From years of working with abalone shell, mother of pearl, and other shell related materials for inlay in guitars, I know.  Yes, it will make you sick, but the damaging part is when the dust particles get in your lungs and cause small cuts, eventually forming scar tissue.  This scar tissue will build over years and eventually lead to decreased lung function. 

On a lighter note, those a cool!

When etching with acid, you can cover the areas that you don't want etched with Vaseline.  It will repel the acid.  I've done something like the abs shown in the picture (years ago) with muriatic acid, then sandblasting in a booth.  As everyone has said, silicate dust (ab shell dust) is bad, bad shit. Use protection! 

I used to carve abalone shells making designs and friends names in the shells.
I got an 18 gallon tub and cut 2 holes in the sides that I could stick rubber gloves through.
I put rope lights on the inside so I could see what I was doing and covered it with a plexiglass top clamped down with weather stripping between the two.
Think automotive parts washer.
I had a dremel tool inside.
I could see what I was doing but kept the dust contained inside of the tub.
I wore a respirator, but having the shell in a concealed environment while I worked on it made a world of difference.

I have a bud that does this in Australia for retail purposes and he uses a 6lb Sand tumbler.
First he soaks the shells in vinegar/water solution, in a black garbage bag, in the sun for a few days. This helps to not stain the shells by over etching the surface with a haze like from muriatic.

Then it's like a rock tumbler for polishing rocks, but in this case you would use black coarse glass sand and tumble it for 48 hours. The results are spectacular, a smooth clean surface and identical to what you have pictured for the finished product, minus many of the direct health hazards.

Cheers,

Geoff

+1

I wonder if you get the same effect with media blasting of some sort...

Sand blaster in a sealed chamber, fine sanding and all of the protective gear mentioned.

sounds like the dust acts like Diatomaceous earth