Steph, on the abalone...I've been told that the ones around the center one have a similar to abalone appearance, but it was caused by being buried. I was told the actual abalone one in the center was made that way off the rollers. It is definitely a little different. It has an almost metallic look, but with an oily rainbow color sheen to it, sort of like looking at light shining on a music CD.
Edit to add nearly 10 years later, that effect was not on the marble off the rollers. All of them are a result of being buried.