I have loved working with stone and exotic woods for about 35 years. It is a fabulous hobby that grew into what I do now.  Pictures are of the actual item/lot shown.It is kind of like fossilized fish eggs and very well may be. It looks like the matrix is agatized or quartzized sandstone. The edges do not have a rhyolite or jasper look to them. Being around in this business a while I can say it must be rare. The only time I ever saw it, I bought it. 6-12 mm thick.  Shown wet. Enjoy and have an exceptional day.