FACT SHEET
Zebra rock and other Kimberley siltstones
by Bruce Livett PhD, Zebra Rock Gallery, Packsaddle Rd., Kununurra, WA
Open 7 days, all year. (08) 9168 1114 www.zebrarock.biz
The highly decorative banded zebra rock is unique to the East Kimberley region of WA. Deposits
have been found at several sites adjacent to and within Lake Argyle. It is not found elsewhere in
Australia or anywhere else in the world. For this reason, it is rarer than diamonds but not yet as
expensive! However, if the water level of Lake Argyle is raised, as is planned for Ord Phase 3
irrigation, then all known sites of zebra rock will become inaccessible.
Zebra rock is a siltstone, a sedimentary rock porous to water. Being siltstone, zebra rock is finer
texture than sand but not as fine as clay. It is approx. 670 million years old, from the Precambrian
era. Consequently there are no fossilised bones or plant material in the rock. The age of the rock is
determined by carbon-dating of fossilised unicellular organisms, algae. The latest research from the
ANU indicates that the rock could be even older, with an average age close to 900 million years.
Zebra Rock consists of a fine grained siliceous argilite (indurated siltstone or claystone) with
rhythmic patterns of red bands or spots contrasting sharply with a lighter background. Examination
of the mineral composition indicates that the principal phases include kaolinite, dickite, quartz,
sericite, alunite and hematite, and that the amount of each varies appreciably even within a single
hand specimen. It is not known how the regular patterns were formed but the red stripes are
coloured ferric (iron) oxide.
Geologists have investigated Zebra Rock without producing any agreed consensus as to its origin.
The origin and formation of the rock is referred to as the “600 million year old mystery”.
Observations by D.R. Mackie and colleagues (Mattievich et al 1990) show that the kaolinite and
hematite particles in zebra rock have a common preferred orientation indicating that the
Precambrian liquid crystal particles were of micrometer size with linear dimensions of tens of
centimetres . Current theories invoke the properties of model ferronematics to explain the patterns
in zebra rock including the segregation of hematite into regular bands and rods, and variations in the
angle between the bands and the sedimentation plane. But who knows what really happened over
600 million years ago. If you have a theory, share it with us. You could be on the money!
We hope you are as enthused as us about the unique zebra rock. The rocks unsurpassed
natural beauty typifies the colour and spirit of the Kimberley.
Other siltstones found in this region include: Liesegang [Weathered Rock] (200 million years old);
Primordial [Ribbon Stone, Okapi] (1200 million years old); Ning Bing (a sandstone) and Astronomite.
Further Reading: Zebra Rock Gallery:
http://www.zebrarock.biz/Geology--Did-you-
know--.html
Zebra Rock Gallery & Cafe (video) http://www.kimberleyexposure.com.au/zebra-rock-gallery-cafe/
Kimberley Siltstones: Zebra Stone - Zebra Rock from Top Rockz http://www.zebrastone.net
Bevan, AWR (2001) Zebra Rock: An ornamental stone from the East Kimberley, Western Australia.
Australian Gemnologist 21: 165-168.
Loughnan, FC and Roberts, FI (1990) Composition and origin of the ‘zebra rock’ from the East
Kimberley region of Western Australia. Australian Journal of Earth Sciences 37: 201-205.
Mattievich, E. et al (2003) Macroscopic ferronematic liquid crystals determine the structure of
Kimberley zebra rock. Proc. 27th Annual A&NZIP Condensed Matter & Materials Meeting, Wagga Wagga.
Parker, K. (2010) Zebra Rock: Jewel of the Kimberley. Australian Gold Gem & Treasure. August p.42.
WA Museum http://museum.wa.gov.au/research/collections/earth-and-planetary-sciences/rockcollection/zebra-rock