When you prep with an air scribe, whenever it hits calcite it will turn the calcite white. In a calcite ammonite, this means you'll be left with a white spiral where you've had to break through the shell of the ammonite in order to prep it - see the picture on the left. You might also have a few white dink marks! Varnish can be used to restore the calcite to the colour it was before prep, and bring back some of the natural shine of the crystal that may have been lost during preparation.