One of chalcedony's most endearing characteristics is it's tendency to form botryoidal (grape-like) structures that manifest internally as "turtle-backing" or sometimes externally as "bubble-top" surfaces. More precisely these formations seem to be features of AGATE..... that is, layered chalcedony. Some of the more massive types of blue chalcedony.....two sources come immediately to mind....one in Turkey and one in Namibia..... do not seem to feature either internal "turtle-backing" or botryoidal surfaces. And even some AGATE material appears to be either too coarsely crystalline or too lacking in sufficient translucency to (visibly) "go botryoidal" although this can vary layer to layer or piece to piece even in material from the same location. These observations are admittedly unscientific and are simply the result of this authors personal experience.
Whatever the cause, when chalcedony "goes botryoidal", a splendid new dimension is added to a stone. I often refer to "turtle-back" as "billowing" within a gem. ... providing extra optical oomph beyond color and clarity. And when (rarely) a surface is decorated by nature with well-preserved botryoidal bumps and bubbles that can be polished to perfection........ ah, sheer bliss!
As a cutter I am particularly attracted to these "bubble-tops". I have developed special tools and techniques that enable me to carefully preserve yet bring to a high polish these botryoidal masterpieces of nature. But appropriate rough with undamaged surfaces is very rare......extremely difficult to obtain from ANY source. I have gone to great lengths to aquire our present stock of bubble-tops. This is the very finest material of this type I've ever had the pleasure to work with....... facet-grade rich blue chalcedony with pristine bubbles. WOW!!