This was the reason for my warning against abalone -
Abalone Shell Chips Causing Brown Stains in Diamond Brite Plaster-
I have a plaster problem
There have been random instances of abalone shells causing brown stains in lighter colored plaster. Abalone is a natural product and cause staining if organic compounds from the shell leaches into the plaster. It seems that there are some high-quality abalone products out there (pre-washed and certified for use in pools) and cheap abalone products. The cheap ones are implicated as the culprit. If you do wish to use abalone, I would sit down with the plaster applicator and share this info with them and ask if they guarantee that they will honor their warranty and replace the plaster if the abalone causes problems.
A better alternative (but likely more expensive than abalone) would be to use clear glass beads to add sparkle. Glass is mostly inert with respect to pool water.
That is an excellent point about the glass versus abalone, Matt. (We used Pebble Tec and their abalone product so it comes with their warranty so I feel confident that if we have problems they will be addressed.)