Silica Replacement

An update for this old Calcite Replacement post.

 After finding another small ammonoid (above) in the same beds, and a discussion on TONMO, I've realized the pattern on the two fossils is that of "Beekite".  Not having a lot of experience with silicified cephalopod fossils around here, I got out the acid bottle, fizz on the matrix, no fizz on the fossil.  So instead of a Calcite replacement, they are replaced with Silica.  Beekite may be a mineral, but the word is more commonly used as a descriptive term for the concentric rings of the replacement material.  

The fossil from the first post showing the "Beekite Rings".