Zebraic chalcedony in a silicified stromatolitic sample in plain light. Same view as the cross-polarized view below. Refer to that for scale.
The black spots are remnant dolomite and some hydrocarbon (probably bitumen).
4x close up - the same crystal as below is 0.125mm
This zebraic chalcedony is thought to replace evaporites (McBride et. al, 1977) (e.g gypsum or halite).
10x close up - the brightest (white) crystal in the center is 0.15mm.
Megaquartz is forming in the middle of the zebraic chalcedony splays.
Thus, the chances are that this stromatolite formed in an upper intertidal environment. There is no other evidence as the dolomitization that occurred before the silicification of this sample has destroyed all other fabric (if) present.
Mcbride, E. and Folk, R. 1977, The Caballos Novaculite revisited: Part II: Chert and Shale Members and Synthesis. Journal of Sedimentary Petrology, v. 47, n. 3, p. 1261-1286.
An addition of cooler chalcedony:
Chalcedony radiating fans of fibrous crystals. 10x magnification in plain light. Same stromatolite as the previous pictures. No idea what it is replacing though as dolomitization occurred prior to silicificaiton.
Same as above picture. 10x in cross polars. Megaquartz filling in the rest of the pore.
Fans are so cool! No pun intended there :D