A gem screw cap holder — commonly called a gem cap in the trade — is a small, cylindrical container comprising a transparent plastic body with a cushioned interior base and a screw-on lid, designed to house a single loose gemstone securely. Inexpensive and ubiquitous, these holders are among the most practical items on any gem dealer's bench, serving equally well for short-term storage, laboratory submission, parcel organisation, and retail counter presentation.

Construction and Materials

The body and lid are typically moulded from clear or lightly tinted polystyrene or acrylic, allowing visual inspection of the stone without removing it. The interior base is fitted with a small pad of foam, velvet, or synthetic fibre that cushions the stone and prevents it from shifting during handling or transit. The screw thread provides a more positive closure than a simple press-fit cap, reducing the risk of accidental opening — a meaningful consideration when the contents may be a stone worth thousands of pounds.

Holders are manufactured in a range of diameters, typically from roughly 20 mm to 50 mm, to accommodate stones from small melee up to large collector specimens. Depth is generally shallow, keeping the stone close to the transparent lid so that it can be examined under a loupe or daylight without extraction.

Uses in the Trade

Gem screw cap holders appear at virtually every stage of the loose-stone supply chain:

• Laboratory submissions: Gemmological laboratories including GIA and Gübelin routinely receive stones in screw cap holders, which protect facets from contact abrasion and allow staff to log and photograph the stone without unnecessary handling.
• Dealer parcels and memo goods: When a dealer sends stones on approval or consignment, individual holders prevent stones from contacting one another and eliminate the abrasion that would otherwise occur in a folded paper parcel.
• Retail presentation: A stone seated in a gem cap on a velvet tray presents cleanly and professionally, and the transparent lid means a client can view the stone under the shop's lighting before the holder is opened.
• Inventory storage: The cylindrical form stacks efficiently in drawers and trays; holders can be labelled on the base or lid with stock numbers, weight, or origin data.

Advantages Over Paper Parcels

The traditional folded paper parcel (briefke in the Antwerp and Israeli trade) remains common for melee and calibrated goods sold by the lot, but for individual stones of any significance the screw cap holder offers clear advantages. The rigid walls protect against crushing; the cushioned base prevents the stone from rocking and grinding against a hard surface; and the transparent lid eliminates the need to unfold and re-fold paper, which introduces handling risk each time. For stones with delicate inclusions, polished surfaces susceptible to contact abrasion, or significant monetary value, the screw cap holder is the more prudent choice.

Limitations

Gem caps are not airtight and offer no protection against humidity or chemical exposure. They are also single-stone containers; sorting a large parcel into individual holders is time-consuming and the holders themselves, though inexpensive individually, represent a cumulative cost at volume. For this reason, melee and commercial-grade calibrated stones are more commonly stored in multi-compartment trays or paper parcels, with screw cap holders reserved for finer or more valuable individual pieces.