The Mount Swite agate locality is accessed from the Bear Creek (Lambly Creek) road via the Hidden Creek logging road that passes approximately 2 kilometres east of the summit.

The agates consist of quartz and cristobalite filling amygdales and fissures in the Attenborough Creek member. The amygdales are commonly elongated almond-shaped structures (0.5 to 5 cm), filled with fine grained blue-grey quartz, cristobalite and white plume opal aligned parallel to flow direction of the lava.

Thunder eggs are larger agates (baseball size) with radiating quartz crystals lining vugs and/or chalcedony in variegated horizontal or concentric bands on cavity floors or walls. Agates are believed to form within gas cavities of volcanic host rocks when microcrystalline chalcedony fibres nucleate on vug walls and grow inward. Oscillatory zoning and iris banding is the result of variations in silica concentrations in solutions at the tips of the growing chalcedonic fibers forming smooth and regular or botryoidal surfaces parallel to the banding (Heaney and Davis, 1995). The most probable source of the silica-rich solutions is the host Attenborough Creek andesite.

Analyses of the andesite from different locations shows uniform composition and excess silica based on norm calculations. It is concluded that part of the excess silica, accompanied by fluids and gases, moved from the andesite lava to gas cavities and fracture openings during the original lava cooling process.