Goodletite: New Zealand's mysterious ruby rock

In November 1891, government geologist Alexander McKay was visiting the gold workings around Rimu and Back Creek when a miner pointed out an unusually hard boulder. McKay took two small specimens to Wellington. By the end of December, analyst William Skey had identified the crystals in the pale-green rock as ruby — corundum.

That discovery began a search that has lasted for more than a century. The rock was real. More boulders would appear. Scientists would eventually explain much of the chemistry that created it. But the characteristic ruby rock itself has remained elusive in its original bedrock setting.

The 1891 discovery near Rimu and Back Creek

The story begins in a landscape already transformed by gold mining. By the 1890s, the first West Coast rushes were decades old, and miners were working gravels that contained fragments of older landscapes. Those gravels carried more than gold: they also carried geological clues from the mountains inland.

On 9 November 1891, McKay was at Rimu with Henry Gordon, the Chief Inspector of Mines. A student from the local School of Mines showed him a sample that had been identified as corundum or emery rock. The following day, in workings around Back Creek and the terrace above the Hokitika Valley, a miner drew McKay's attention to a boulder remarkable for both its unusual material and its hardness.

McKay brought pieces to Wellington, where Skey identified the enclosed crystals as ruby or corundum. The laboratory result was never formally entered into the departmental books — a small archival gap that later had to be explained in McKay's published account.

Why is it called Goodletite?

William Goodlet was travelling among West Coast schools of mines when he obtained samples of the ruby-bearing stone and took them to Dunedin. There they reached Professor George Ulrich at the University of Otago School of Mines. The rock's informal name ultimately became associated with Goodlet, although the discovery itself involved a chain of miners, students, analysts and geologists rather than one single discoverer.

Ulrich's 1894 description recorded ruby-bearing material from a Back Creek boulder weighing about 40 pounds — roughly 18 kilograms. Some of the crystals were finely coloured; others were dark, fractured or dull. From the beginning, Goodletite sat in an unusual space between gemstone excitement and geological reality: genuine ruby did not necessarily mean gem-quality ruby.

The search for the source

McKay took the question into the ranges. He suspected a connection with a belt of olivine-rich rocks and serpentinite — part of what is now known as the Pounamu Ultramafic Belt. If fragments of ruby-bearing rock could be traced upstream, perhaps the original outcrop could be located.

But the river searches did not deliver the hoped-for trail. McKay found garnets, but no ruby. Later investigators found tantalising clues: ruby-bearing boulders in Olderog's Creek, an inferred source around McArthur Crags, and reports from the Whitcombe Valley. Yet loose boulders are not the same as ruby rock demonstrably attached to the bedrock where it formed.

The West Coast landscape makes that distinction especially important. Glaciers can carry rock far from its source; rivers can then rework old glacial deposits and move the same material again. A boulder found in a creek may therefore have travelled through several landscapes before reaching the place where someone finally notices it.

What is Goodletite made of?

The simple description "ruby in fuchsite" captures the colour but not the full rock. Goodletite is not one mineral. It is a rock containing corundum — ruby and sapphire — within a green matrix that can include more than one mica. Chromium-bearing muscovite, commonly called fuchsite, is important; so is margarite, a calcium-rich mica. Accessory minerals include chromite and tourmaline.

A major clue came from large sectioned boulders. One important specimen showed a ruby-and-sapphire-rich centre, an emerald-green mica zone, chlorite farther out, and a surviving remnant of serpentinite at the edge. That outer rind mattered because it connected the spectacular corundum-rich interior to the ultramafic rocks long suspected in the broader geological story.

How could ruby and sapphire form in the same rock?

Work by Rodney Grapes and Ken Palmer treated the rock as the product of reaction rather than a simple cavity filled with gemstones. In their model, schist enclosed within serpentinite underwent intense chemical exchange involving hot fluids. Some components were removed, and aluminium became concentrated in the altered rock. Under the right conditions that aluminium formed corundum.

The textures show that minerals replaced one another through time. Red and blue regions can occur within the same corundum crystal. Chromium is associated with ruby's red colour, while iron and titanium can contribute to blue sapphire. Later reactions could attack the corundum again, producing still more minerals.

This makes Goodletite valuable scientifically as well as visually. A single polished slice can preserve multiple stages of chemical change, while the weathered outer zones can contain clues to the environment in which the rock formed.

Has the original Goodletite source been found?

The safest answer is: not conclusively in the published history used for this episode. Candidate areas have been proposed and ruby-bearing boulders have been found closer to the mountain belt, but the characteristic rock has not been securely documented still attached to its original bedrock source.

That unresolved point is part of what makes Goodletite so compelling. Scientists can reconstruct much of the rock's deep chemical history while still being unable to point to the exact patch of mountain where the classic boulders formed.

Why provenance matters

For collectors, a Goodletite specimen has two kinds of history. One is written into the minerals: corundum, mica, reaction textures and surviving outer zones. The other is human: where the specimen was found, which boulder it came from, who cut it, and what information stayed attached to it.

A beautiful slice without a record remains beautiful. A documented slice can also become evidence.

Listen to the full story

The podcast episode Goodletite: The Hunt for Ruby Rock follows the discovery, failed searches, later boulders and mineralogical work in narrative form.