Zebra stone is an interesting sedimentary stone which comes out of Australia. It can be hard to distinguish it from the other trade names, however, so most rockhounds will need a little bit of clarification before digging too much deeper.
So, let’s take a closer look at zebra stone, and discuss where to find it. We’ll even include some tips for lapidaries who want to work with this material!
What is Zebra Stone?
Zebra stone is a trade name, and as such it’s notoriously imprecise. In this case it usually refers to a specific type of siltstone that comes from Australia. This stone bears distinctive red and white banding in very clean lines, reminiscent of the stripes on the side of its namesake animal.
It may also refer to zebra marble. Zebra marble is a metamorphic stone that’s also generally sourced from Australia but deserves its own treatment in a different article. It’s quite distinct from the sedimentary variety.
And, just to make things nice and complicated, there are also dozens of different stones called by variations of zebra jasper and zebra agate. These stones are generally colored chalcedonies that happen to have stripes resembling a zebras, although they come in many colors. Be aware that the names agate and jasper are also used for rhyolite and other rocks because people can slap a name on a stone without understanding what it is.
What makes zebra stone distinct is that it generally has clean lines, unlike most banded stones. The distinction between the red and white areas is very distinct, which is a rarity in stones of all types. The patterns tend to be repeating in the best samples, occurring in the same size at regular intervals.
Siltstone is a rock comprised of fine-grained quartz particles. Specifically, silt is defined as particles between sand and clay. In practice, this works out to being .032mm and .007mm in size for the most part.
There’s a wide range of other elements included in the composition of the rock, including iron oxide which appears to be the coloring agent for the red portion of the stone. Others contribute to the stone’s texture and overall appearance.
The different patterns are distinct from each other, and some are incredibly rare. The creation of Lake Argyle covered much of the area with known deposits, and some patterns have their entire yield measured in kilograms instead of tonnes. It appears in relatively thin seams between layers of shale that form muchn of the rock in the area.
Extraction must be done by hand, making it a time consuming and rather expensive process. In some cases, formations have been saved from mining for future generations to enjoy.
How Was Zebra Stone Formed?
Well… no one really agrees. While that’s a fairly common thread in geology, in this case its a mystery that has been studied for decades.
What we do know is that the stone formations appear to be about 600 million years old. It’s likely that the alternating layers of seasonal silt were deposited over time before being crushed underneath the weight of the earth, leading to the alteration of the stone. The stone’s surface is much like unglazed porcelain. This is because it is exceptionally fine-grained even for a siltstone.
Since the red color seems to come from iron oxide, it appears that there must have been some sort of rhythmic precipitation of iron oxide. This is also the mechanism proposed for the banding in some agates, although it’s still up for debate.
This is one case where the formation is still actively being studied, so new information si sure to lead to a more solid theory in the future. At the moment, however, it remains something of a mystery how such clean lines and repetitive patterns showed up in the rare seams of zebra stone.
Where is Zebra Stone Found?
Zebra stone is found in Kununurra in Australia. As noted above, the creation of Lake Argyle submerged much of the known zebra stone deposits in the area but it’s still pulled from the earth in quarries around the region.
This seems to be the only place it occurs.
The formation itself is known as the Ranford Formation. Zebra stone occurs in the shale formations of this area as small, isolated “reefs” of rock. Often the pattern isn’t distinct from the outside, instead the stone must be cut to discover what lies on the interior.
Ornamental Stones from the Ranford Formation
Zebra stone isn’t the only ornamental rock which emerges from this area. There are a few other distinct types of stone that can be found. While similar in some ways, they’re actually quite distinct and worth looking into if you’re interested in stones for carvings and other ornamental applications.
- Astronomite- This is a variety of claystone, colored by hematite. Reactions within the stone created small spheres that are pinkish in color on a darker background. These are randomly distributed throughout the material and astronomite is actually the rarest of the Ranford stones.
- Okapi Stone- Colored by parallel patterns of some length, generally with black, brown, red, and pink tones. Its distinct from zebra stone, since the colors tend to blend together instead of having the clear definition found there.
- Ranford Ribbon Stone- Possesses irregular banded patterns,aa in contrast to the repeating ones found in okapi and zebra stone. The tones are generally grey, blue, orange, and reds. The stone itself usually has a general appearance of grey and black, with bands of the other colors intersperesed at irregular intervals.
- Primordial Stone- Similar to zebra stone in composition, but generally containing lens-like inclusions instead of regular lined patterns. This is the hardest of the Ranford ornamental stones currently being mined.
Tips for Working With Zebra Stone
It’s important to remember, when approaching work with zebra stone, that it’s best considered a soft stone. Similar stones to work would include very compact limestone, and even marble.
This is distinct from the hardstone that most of us are used to working with. My background, for instance, is primarily in hardstone carving which includes things like jasper, agate, and the various types of quartz.
Softstone carving is a bit different, especially since you can mainly work with steel tools. This means that different techniques are possible, but some of the usual logic doesn’t apply. For instance, “normal” lapidary tools aren’t of much use here since the fine grains can clog the usual grinding tools quite easily.
Most of the Ranford Formation ornamental stones are soft enough to be cut with a hacksaw or other hardened steel saw. You can still use a lapidary saw for this, and that’s generally how it’s blocked or cut to dimension before sale.
If you’re planing on making cabs from this material, which is sometimes done, it’s best to start with a much higher grit than you’d typically use for the usual suspects. Using a 400 grit wheel to rough out the cabochon is a good idea.
The softness of the stone means that it will be hard to get it to take a real polish and a mirror polish is going to be impossible to achieve. Instead, try taking it off the grinding wheel and moving upwards incrementally with different grits of wet sandpaper. Somewhere between 1500 and 2500 will probably be the best you can do, no need for attempting to use a “real” polishing compound.
Carving for sculptures can be undertaken in the same ways as any traditional softstone, with either hand powered or pneumatic chisels depending on your skill set and what tools you have in the first place.
