Montana agate structure









Rock Hunting







Banjos, etc.


Village of Baoma,

Sierra Leone









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If you throw a white t-shirt into a pot of dye and let it steep overnight, it will uniformly and homogeneously acquire the color of the dye.  If you first tie the t-shirt into knots and then throw it in the pot of dye for a short period of time, you get distinctive "tie-dyed" color patterns.  This occurs because the t-shirt fabric is compressed at the knots, and less permeable to water and dye.  Dye enters the fabric and selectively penetrates the more permeable areas.

Another laundry-room example........ if a pen leaks in the pocket of a white shirt, the ink will spread out into the fabric from the point of origin.  If the shirt is slightly wet, you may even see slight differences in color of the ink stain, due to the ability of some constituents of the ink to move (with the assistance of water) faster than other constituents through a specific, porous medium (in this case, the shirt fabric) .  This principle has been developed into the analytical science of chromatography.

Both of these examples relate to how the internal structure of Montana agate affects the formation and evolution of colored inclusions.

Porosity                                        top of page

Hydrogeologists describe porosity in bedrock as being primary (porosity between individual grains in the rock) or secondary (porosity in cracks and crevices in the rock).  Some would also add tertiary porosity, or porosity in large open spaces (such as caves or solution channels in karst berock).  Groundwater circulation through bedrock is dominated by secondary and/or tertiary porosity features, with much slower movement of water through primary porosity features in intact bedrock.

The same hydraulic principles apply, on a much smaller physical scale and over a longer time scale, to the circulation of water through a Montana agate. 

Heterogeneous Primary Porosity                       top of page

If you look at a photomicrograph of clear agate, you will see zones of orientation of individual crystals of quartz that correspond to banding (fortification) features, even although the banding may not be visible to the eye.  The banding features are likely the result of the gradual, depositional process that created the agate.  This heterogeneity means that water may have a preferred direction of movement through primary porosity features.  As a result, water-related alteration of an agate will not be uniform and homogeneous throughout the rock, but will tend to be influenced by the texture of primary porosity features.

Whether originating from replacement of plant matter (limb casts) or from in-filling of other spaces in the subsurface, most Montana agate appears to have had a cylindrical or elliptical shape when it originally formed.  Today, some agates (especially smaller agates) appear have preserved something close to the original form.  Other agates are clearly broken down from a larger, original stone.

Secondary Porosity                               top of page

Secondary (2') porosity in agate can be described as interconnected void spaces consisting of cracks or crevices in the rock.  Water will enter an agate through 2' porosity features and may then disperse into the rock adjacent to the 2' porosity features.  When water drains from the immediate area in which an agate is situated, then water will exit the agate primarily through 2' porosity features, with drainage towards the 2' porosity feature from adjacent, uncracked rock.

Capillarity                                    top of page




 Central cavities


chromatographic separation



cracks & checks

Botryoidal surfaces, relic fortifications




It's common to find sucrose or coarse crystalline quartz, or perhaps even a void space in the center of an agate nodule, surrounded by agate @ botryoidal surface boundaries. 

Very often see "fortifications" that seem to follow bot surfaces..... possible slow, phased  replacement of coarse/sucrose by chalcedony ?  Also common to see relic fortification banding & relic bot surfaces

Ribbon banding often seems to follow relic, botryoidal surfaces or else fortification banding.  Ribbon banding is commonly brown, sometimes orange to red.  Red ribbon banding seems to progress towards diffuse red, brn ribbon banding often seems to transition towards formation of dendrites.

1' porosity

2' Porosity features

Find a lot of petrified wood, appearance often seems to grade from pet wood (w/ banding from growth rings) to ribbon banded.







ribbon banding follows the bot surfaces or else fortif banding, commonly brown, sometimes red.

3-d, floating dendrites are almost always black.  Sometimes thin dendrites are reddish.






Readily visible, crystalline quartz may have a roughly hexagonal, prismatic shape,


or it may be composed of tiny, discrete crystals (granular, or  "sucrose" form),


or tiny, interlocking crystals (massive, or "bull quartz").


readily visible crystals often contain inclusions (such as minerals that were trapped as the crystal grew).




Chalcedony can be divided into opaque forms (jasper) and translucent forms (agate).  Agate has a number of distinctive, characteristic forms:


botryoidal (grapelike),

mammillary (breastlike),  
banded in parallel, irregular shapes (fortification agate), and  
banded in flat, parallel, patterns (onyx)  









coarse crystalline


phantom Bot

fortification banding


healing of former fortification banding.


Botryoidal habit


Finding Montana Agate