A cylindrical hollow tube is an elongated, channel-like inclusion formed within a gemstone's crystal lattice during growth. Appearing under magnification as a dark, linear feature with sharply defined walls, these inclusions run parallel to the host crystal's c-axis and may be entirely empty or partially to fully filled with fluid. They are classified as negative crystals — voids that mirror the geometry of the surrounding crystal structure — and are documented as diagnostic inclusions in several gem species, most notably Sri Lankan alexandrite and certain beryls.

Formation and Structure

Cylindrical hollow tubes originate during the primary growth stages of a crystal, when localised disruptions in the growth environment — fluctuations in temperature, pressure, or fluid chemistry — prevent complete infilling of a channel along a crystallographic direction. The result is a hollow conduit whose walls conform to the crystal's internal symmetry. Because the tubes align with the c-axis, they typically appear as parallel arrays when viewed along specific orientations, and as single points or short rods when viewed end-on. Their walls are often optically smooth, giving them a reflective quality that distinguishes them from solid mineral inclusions or fractures.

Where fluid is present, it may be a primary fluid trapped at the time of crystallisation, offering potential insight into the geochemical conditions of formation. In some specimens the tubes are partially healed, with the fluid occupying only a segment of the channel.

Occurrence in Sri Lankan Alexandrite

Cylindrical hollow tubes are among the most characteristic inclusions of alexandrite from Sri Lanka (historically known as Ceylon), where the gem-bearing metamorphic and alluvial deposits of the Ratnapura and Elahera regions have produced alexandrite for centuries. In Sri Lankan alexandrite, the tubes frequently occur in dense, sub-parallel groupings that can impart a silky sheen to the stone when oriented correctly — an effect occasionally exploited in cabochon cutting. Their presence, particularly in combination with other Sri Lankan indicators such as rutile needles and zircon crystals with tension halos, forms part of the inclusion fingerprint used by gemmological laboratories to assign a geographic origin determination to the stone. The Gübelin Gem Lab and GIA both document these features in their origin reports for alexandrite.

Occurrence in Beryl

In beryl — the species encompassing emerald, aquamarine, heliodor, morganite, and goshenite — cylindrical hollow tubes have been recorded in specimens from several localities. Aquamarine from Brazil and certain African sources may display fine tubes running along the length of the crystal, sometimes visible to the naked eye in larger rough material. In emerald, the inclusion environment is typically more complex, but tube-like channels have been noted as secondary features alongside the more characteristic three-phase inclusions and growth veils. The tubes in beryl are generally consistent with the hexagonal symmetry of the species and align with the c-axis as in other host minerals.

Gemmological Significance

The primary value of cylindrical hollow tubes to the practising gemmologist lies in their diagnostic utility. Because their morphology, orientation, and association with other inclusion types are characteristic of specific geological environments, they contribute meaningfully to origin determination — a factor of considerable commercial importance for fine alexandrite, where Sri Lankan origin commands a premium over material from Brazil, India, or East Africa.

In terms of clarity, isolated or fine tubes have a limited impact on a stone's appearance and transparency. Dense concentrations, however, can reduce transparency, introduce a milky or silky optical effect, or — in extreme cases — lower a stone's clarity grade to a commercially significant degree. Gemmological grading laboratories assess tubes alongside all other clarity characteristics when assigning a grade, weighing their size, number, position, and visibility to the unaided eye.

Cylindrical hollow tubes do not compromise the structural durability of a gemstone in the way that surface-reaching fractures or cleavage planes might; being enclosed channels, they present no direct vulnerability to chemical infiltration during cleaning or setting.

Identification Under Magnification

Under a standard binocular microscope at 10× to 40× magnification, cylindrical hollow tubes appear as dark, well-defined linear features. Oblique fibre-optic illumination is particularly effective in revealing the reflective inner walls of empty tubes, which may flash brightly as the angle of light is adjusted. Darkfield illumination renders fluid-filled tubes as luminous channels against a dark background. The distinction between a hollow tube and a healed fracture or a solid needle inclusion rests on the tube's consistent circular or near-circular cross-section, smooth walls, and alignment with the crystallographic axis of the host.

Further Reading

• GIA — Inclusions in Gems
• GIA Gem Encyclopedia — Alexandrite