The Gem is the smallest and most mysterious of the Thomson Bay wrecks. She was a 52-ton wooden cutter that went down with all hands - four passengers and six crew - while approaching Rottnest on 18 May 1876 with a cargo of wheat from Port Irwin. The cause of the loss has never been explained, and there have long been suggestions that the story around her sinking and salvage was not fully told.

Very little remains. The WA Museum trail describes the wreck as lying on sand with some ribs and timbers still showing, while divingwawrecks notes only planking and nails around a museum plinth placed at the site roughly 100 m east of the Janet. How much is exposed depends on recent sand movement. The Museum also notes that old rope and rigging in the area can snag divers.

This is a site for wreck-history enthusiasts and people completing the Rottnest wreck trail rather than for a scenic dive; the attraction is standing on the spot where ten people disappeared within sight of the settlement. It lies within the Kingston Sanctuary Zone and is protected; nothing may be disturbed. Most divers combine it with the Janet, Macedon and Denton Holme in a single shallow circuit of Transit Reef. For the best chance of seeing timbers, visit after a period of calm easterly weather, when winter sand has had time to scour away, and carry a torch to pick out the dark planking against the pale sand. Navigation across open sand between the Transit Reef wrecks is the main skill the site teaches, so it is a useful exercise for newer divers planning to do the whole circuit with a compass.

Rottnest lies about 18 km west of Fremantle. Perth charter boats (Perth Scuba's Blue Destiny and Bucket List Diver) leave Fremantle in the morning and reach Thomson Bay in about 45-60 minutes; the Transit Reef wrecks are usually picked up on a mooring just outside the bay as a second, shallow dive. Private boats launch at Fremantle, Woodman Point or Hillarys and should use a public mooring rather than anchor inside the Kingston Sanctuary Zone.

It is also possible to catch the Rottnest Express or SeaLink ferry to the main jetty and kayak or make a long surface swim north from the Thomson Bay beach, but the route crosses a busy boat channel and a tow float and flag are essential.

Protected from the south-west but exposed to north-westerly winds and winter storms; best in under 2 m of swell on calm mornings. Sand movement buries and exposes the timbers seasonally. General pattern at Rottnest: summer brings calm morning easterlies followed by a strong south-westerly sea breeze from late morning, so dive early; winter brings north-westerly fronts and large swell, with clear, calm spells between them.

Check the Bureau of Meteorology coastal forecast for swell height and period before committing.

Anchor on sand near the plinth, descend and search slowly around it for exposed ribs, planking and nails; the site is small and can be covered in 15-20 minutes before swimming west to the Janet's anchors. Snorkellers should stay within easy reach of the exit point and swim with a buddy; scuba divers at these depths need no safety stop but should ascend slowly and surface near the float.

No regular dive operator - occasionally included on semi-submersible tours of the Thomson Bay wrecks; divers visit from private boats or as an extension from the Janet.