A fantasy cut attachment — also termed a concave attachment — is a specialised accessory fitted to a standard precision faceting machine that enables the lapidary to grind and polish concave, or inwardly curved, facets into a gemstone's surface. Where conventional faceting produces only flat, planar faces, the fantasy cut attachment introduces cylindrical or spherical depressions that interact with light in fundamentally different ways, scattering and refracting it along curved paths and producing the layered, three-dimensional optical effects characteristic of the fantasy cut style pioneered and popularised by German master cutters — most notably Bernd Munsteiner — from the 1960s onwards.

Mechanical Principle

The attachment replaces or supplements the standard flat lap of a faceting machine with a rotating arbor carrying a small cylindrical or barrel-shaped grinding tool. This tool spins on its own axis — typically driven by a separate motor or a flexible shaft connected to the machine's drive — while the gem, held in the faceting head's dop stick, is presented to the rotating surface at a controlled angle and depth. The combination of the arbor's rotation and the lapidary's controlled lateral or plunge movement carves a smooth concave depression rather than abrading a flat plane. Polishing is achieved by substituting the grinding arbor for one charged with a polishing compound appropriate to the gem material in question.

Construction and Compatibility

Most commercially available fantasy cut attachments are designed as bolt-on or clamp-on units that mount to the mast or base of standard faceting machines, including widely used platforms such as the Graves, Ultra Tec, and Facetron. The arbor tools themselves are available in a range of diameters and profiles — narrow cylinders for tight, precise channels, wider barrels for broad sweeping concavities — and are surfaced with diamond abrasive in grits ranging from coarse stock removal (typically 260 to 600 grit) through to pre-polish and polish grades. Some lapidaries fabricate their own arbors from brass or aluminium rod, charging them with diamond paste, which allows bespoke profiles not available from commercial suppliers.

Because the attachment introduces a rotational axis perpendicular or oblique to the gem's presentation angle, precise depth control is critical; most units incorporate a depth-stop mechanism or micrometric feed screw to prevent over-cutting, which in a transparent gem would be immediately visible and irreversible.

Application in Fantasy and Freeform Cutting

The attachment is the defining tool of the Phantomschliff (fantasy cut) tradition, in which concave facets are combined with conventional flat facets, free-form outlines, and sometimes internal carving to produce sculptural gemstones that function as much as miniature optical instruments as jewellery components. Suitable materials are those with sufficient hardness and toughness to accept the lateral stresses of concave grinding without fracturing: quartz varieties, topaz, tourmaline, citrine, smoky quartz, and certain feldspars are common choices. Harder materials such as sapphire and spinel can be worked with appropriate diamond tooling, though the time investment increases substantially. Softer or more included stones are generally unsuitable, as the curved surfaces amplify the visibility of inclusions and cleavage traces.

The optical consequence of a well-executed concave facet is a broadening and softening of light return: rather than the sharp, mirror-like reflections of a flat facet, the curved surface distributes reflected and refracted light across a wider angle, creating a sense of depth and luminosity that many collectors and designers find compelling in large statement stones.

Skill Requirements

The fantasy cut attachment is firmly in the domain of advanced lapidary practice. Its use requires a thorough understanding of gem optics, precise hand control, and the ability to visualise how a curved depression will interact with the stone's existing facet geometry and crystallographic orientation. Unlike flat faceting, where the machine's angle and index settings largely govern the outcome, concave work involves a significant freehand component; consistent results come only with considerable practice. Most lapidary curricula treat concave cutting as a post-intermediate or advanced module, introduced after the student has mastered standard brilliant and step-cut forms.

In the Trade

Gemstones cut with a fantasy cut attachment command a significant premium over conventionally faceted stones of equivalent material, reflecting the additional time, skill, and tool wear involved. In the collector and designer market, named cutters working in the fantasy tradition — particularly those trained in or influenced by the German school — achieve prices that are largely independent of the underlying gem material's intrinsic value, with the cutting itself constituting the primary source of worth. Auction houses and specialist dealers typically describe such stones as fantasy cut, concave cut, or attribute them to the Munsteiner cut tradition when the lineage is clear.