Mamillary habit (also spelled mammillary, from the Latin mamilla, breast) is a descriptive term in mineralogy for crystal aggregates that form rounded, smoothly curved, intersecting hemispherical surfaces resembling a series of low domes or small breasts. It is closely related to, and often used interchangeably with, botryoidal habit, though strict usage distinguishes the two: botryoidal aggregates resemble bunches of small grapes with surfaces of relatively small radius, while mamillary aggregates display larger, smoother, more flattened curves with broader radii.

The habit develops through the deposition of mineral material from solution onto an irregular substrate, with crystallisation proceeding outward radially from numerous nucleation points. As adjacent rounded growths expand and meet, their curved surfaces intersect and remain visible as a continuous, undulating exterior. Internally, mamillary aggregates often display radial fibrous or columnar texture revealed when the specimen is cut and polished — a structure that is itself diagnostic and aesthetically valued.

Common gem and mineral examples

The habit is encountered in a range of gem and ornamental materials. Hematite famously occurs in mamillary form at numerous localities; the lustrous, kidney-shaped masses from Cumbria and from Minas Gerais are classic specimens that gave rise to the variety name kidney ore. Malachite from the Democratic Republic of Congo, Russia and Australia displays mamillary growth that, when sliced, yields the concentric banded patterns prized for ornamental work. Goethite and limonite often form mamillary crusts. Smithsonite, calcite, chalcedony and certain forms of agate and opal also adopt the habit under appropriate solution conditions.

For the lapidary, mamillary specimens offer two faces of value. The unworked exterior surface displays the curved, satin-lustred dome structure that collectors prize for mineral cabinets, while sectioning reveals interior banding or radial fibres that can be polished into striking cabochons or slabs. Malachite is the prime example: a mamillary mass cut perpendicular to growth produces the characteristic concentric ringed patterns familiar from Russian czarist hardstone work and modern decorative carvings.