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Throughout the Herkimer Mining District there are faults (large cracks in the Earth where the land mass on one side of the crack shifted relative to the land mass on the other side).  These faults pass through the birth place rocks of Herkimer diamonds and it is likely some were active while those rocks were being laid down. Geologists have found zones of dolomite around these faults, where the original calcium carbonate limestone has been altered, changed to a magnesium-calcium carbonate = dolomite. The process of doing this results in a change in volume within the rock and thus an increase in the porosity (more holes). The first dolomite event may have been wide spread around the faults (both here and in the gas well geology of the Finger Lakes). The crystals from this early event are very small.  See the dolomite page:

The nature of mining for Herkimer diamonds is mostly a near surface process, and usually at fairly shallow depths (10-15 ft). The two mines in Middleville, which supply a good majority of market place Herkimer diamonds, have a little more rock exposure (20 to 50 ft). It appears from this limited rock exposure that the mineral bearing zones in these mines are horizontally controlled.  But is this true across the entire district? Are ALL the deposits in the Herkimer district are strictly horizontally controlled? It is a reasonable belief given the sampling bias created by the surface mining conditions that limit our vision of vertical structures. But this view changes once you have more vertical surface to examine - as in the HBQ site - where we have large vertical rock exposure (hundreds of feet) and can see a clear relationship between vertical structures and the larger pockets (some big enough for a man to sit in comfortably).  Then add to this observation of the region wide fault controlled first dolomite, the fault influenced gas deposits over 100s of feet in vertical height, and the fact that many Herkimer deposits are near escarpments (cliffs) yields a conclusion that story about how Herkimer diamonds are made has to be linked to a fluid plumbing system that has both vertical and horizontal flow - and this is a critical part of understanding the diversity of mines, and mineral features, across the entire Herkimer Mining District.  For more on the vertical and horizontal flow features visit the page:

After the first dolomite event, dolomite was part of the crystal forming process that made Herkimer diamonds, and crystals of dolomite may have been forming and dissolving during the making of Herkimer diamonds.