INVESTIGATING COMPLEX ETCH CHANNELS IN NATURAL DIAMOND

Etch channels are rare inclusions that have been reported in all natural diamond types. Non-linear (“worming”) channels are believed to be true dissolution features, but the nature of the etchant and the structures exploited have yet to be experimentally confirmed. The authors recently had the opportunity to study a faceted pink diamond containing numerous etch channels. Gemological observation revealed a complex scene of branching and convoluted channels with polygonal cross-sections and stepped walls. The channels were documented using digital microscopy, scanning electron microscopy, cathodoluminescence imaging, micro-CT Imaging, and photoluminescence mapping. Although this documentation does not point to a concrete pathway of formation, this particular sample offers a unique insight into the types of defects that may cause these features to form.

Studies of dissolution morphology in natural and CVD diamond demonstrate that lattice defects create localized strain that is more vulnerable to chemical attack; as such, worming channels may be empty “casts” of long-range lattice defects that have been preferentially dissolved. A strong candidate for such an extended defect is a surface-reaching threading dislocation: a missing chain of atoms propagating from local strain fields between crystal and substrate during epitaxial growth. Transmission electron microscopy (TEM) has linked individual etch pits to threading dislocations in both CVD diamond and natural diamond crystals, and diamond anvil experiments involving chloride-enriched aqueous fluids at high P-T have created incipient etch channels resembling those found in natural diamond.

Another possible formation mechanism involves twinning-induced Rose channels being subsequently etched by mantle fluids. This would eliminate the problem of worming channels originating on plane faces, which are energetically unfavorable locations for etching under normal mantle conditions. Rose channels could provide etchants a pre-existing path into the diamond interior, where they then followed the geometry of the threading dislocations. As most pink and brown diamonds contain micro-twins, but rarely worming channels, the specific P-T and chemical conditions required for this mechanism would be uncommon and require further exploration.