This specimen is a fine cluster of fishtail gypsum crystals collected near Bolton, Victoria, Australia, during the 1970s. Measuring 13.0 × 7.7 × 4.8 cm and weighing 352 grams, it comes from the A & G McDonald Collection, giving it both provenance and historical value. The “fishtail” or twinned crystal form is one of gypsum’s most recognisable and desirable habits, noted for its elegant symmetry and striking natural geometry.

The crystals are sharply defined, with the characteristic V-shaped twinning that gives fishtail gypsum its name. Their surfaces are smooth and glassy, with a subtle translucency that allows light to penetrate and emphasise their internal clarity. The cluster is well balanced, with multiple twinned crystals intergrown, creating a sculptural and highly aesthetic natural form.

Gypsum is a calcium sulfate mineral that forms in evaporitic environments, often through the slow crystallisation of mineral-rich waters. Fishtail twinning occurs when crystals grow in symmetry along a shared plane, producing distinctive “swallowtail” shapes that are sought after by collectors. Australian gypsum, particularly from localities such as Victoria, is prized for its well-defined twinning and excellent display qualities.

This specimen is a fine example of Australian gypsum mineralogy, combining excellent crystal form, desirable fishtail twinning, and historical provenance. Its size and aesthetic balance make it a display-worthy piece for any collection of evaporite minerals, Australian classics, or twinned crystal forms. The link to the A & G McDonald Collection further enhances its value as a collectible specimen.