TumbleStone

Today’s fossick was partly prompted by a fellow rockhound turning up at Chrissy’s with an interesting blue stone. Michelle operates Deep South Pounamu (Mataura) and is a member of the Southland Geological and Lapidary Club. She had been looking for stones on Gemstone Beach with her daughter Amelia. They had found quite a few hydrogrossular garnets and, in the same spot, came across a small heavy blue stone that was unlike any hydrogrossular she had seen before. I took some photos but then accidentally deleted them. Michelle sent me these photos later:

Some hydrogrossular garnets found by Michelle and Amelia, with the blue stone in the centre. Photo by Michelle.

Could this be a Gemstone Beach sapphire? Photo by Michelle.

Close-up.

I noted at the time that the stone did not have the deep royal blue of the Gemstone Beach sapphires I had previously viewed. More on this stone at the end of this Post.

Prompted by Michelle and Amelia’s hydrogrossular garnet finds, Chrissy and I felt it would be a good idea to visit Gemstone Beach this afternoon. The weather was not the best. We arrived at 3pm in unpleasant conditions, just over an hour past high tide. The temperature was 12 degrees, there was a cold wind blowing, and storm clouds were overhead. There was a high swell and lots of foam on the beach. There were a handful of hardy souls there, including one of the local small-scale gold miners.

There were 3 or 4 people by the Stream With No Name.

A lot of foam was blowing about in places.

Chrissy and two other brave stone hunters.

The waves were too high to go past the Taunoa Stream.

The waves were sweeping in back and forth over a good patch of stones in front of the car park. We walked down to the Taunoa Stream but decided not to attempt a crossing – the stream was deep and swiftly flowing, and the occasional wave was coming in nearly to the foot of the cliffs. Sand had been deposited in this area and there were no stones to be viewed. So we went back and spent our time looking in front of the car park, often backing off from the higher waves. There were lots of interesting stones there. 

Overall, we collected some good specimens though we found very few hydrogrossulars. Ten of my finds are featured below.

One of the prettiest stones I found was a small amygdaloidal one:

Small amygdaloidal basalt.

Other side of stone.

Tiny holes originally filled by gases as the basalt was cooling have been infilled by colourful minerals such as zeolites. See “Z is for Zeolite” for more information  on amygdaloidal stones.

A dark red stone with lots of texture when viewed up close:

A nice small light-green quartzy stone, probably brecciated – I find these very attractive and interesting:

Next is a small stone with light coloured patches standing slightly proud against a dark red/purple background. When I picked it up, I could see tiny hints of green and light red:

An igneous stone with white crystals, this stone drew my eye because the crystals seemed especially bright:

For examples of variations of such stones, see Stones W43 to W63 in this Post. 

I picked up a stone that felt rough, like a sandstone. But its grains appeared to be larger than I expected. When photographed and zoomed in, you can see a fascinating bunch of teeny tiny fragments:    

I spotted quite a few trace fossil stones, a couple of which were unusual. This one in particular is very attractive. Its traces are the same colour as the host rock, and are particularly well defined:   

Two other argillite stones with trace fossils:

Some argillite stones have patterns in them that may or may not be trace fossils. Here is an example:

Argillite.

To return to Michelle’s blue stone mentioned at the start of this Post. She later send me the results of some testing she had just done, comparing it to three hydrogrossular garnets. She checked their fluoresence and specific gravity, and the blue stone’s hardness. Her concern was to see if the blue stone is a hydrogrossular or might in fact be corundum, the material that makes up sapphires.

Fluorescent stones glow in visible colors when exposed to ultraviolet (UV) light because certain trace inclusions absorb and re-emit the energy. Michelle has found that hydrogrossular garnets tend to look pink under uv light, something that Chrissy and I also observed when we visited Gemstone Beach one night with uv torches. This website suggests that only some hydrogrossular garnets fluoresce. Michelle found that her blue stone reacted very differently to uv light compared to her other three. She wrote in her summary of her testing: “Under the same UV light, Stone #1 [the blue one] showed little to no visible reaction, while the comparison stones identified as hydrogarnet showed a noticeable reaction.” From what I can tell online, blue corundum tends not to fluoresce. 

The specific gravity of a stone is its relative density, a measure of mass by volume. As one local website puts it, a stone’s specific gravity is the ratio of its mass to the mass of an equal volume of water. One way of calculating it is as follows: you weigh the dry stone, then suspend it completely in water sitting on a scale, and see how that changes the weight measured by a scale; you then divide the dry weight by the extra weight of the water in which the stone is suspended. The process and why it works keeps eluding me from time to time – I found this video useful:

Michelle used this method to calculate the specific gravity of her blue stone and the three hydrogrossular garnets. These are her results:

Michelle found her blue stone to have a higher specific gravity than the three hydrogrossular stones. Note that any or all of these stones could contain material other than hydrogrossular which will affect their specific gravity measure.

Generally speaking, there are accepted standard specific gravity measures for minerals. Corundum, for example, is said to have a very high specific gravity of 4.0, though Wikipedia notes that it can vary between 3.95 and 4.10. Quartz has a specific gravity of 2.65 and diamond is 3.5. A rock that is predominantly one mineral can have a little bit of other material in it, decreasing or increasing its specific gravity a little.  Most rocks are made up of two or more minerals and their specific gravity will vary depending on the precise mix. 

I did an online search for the specific gravity of hydrogrossular garnet. Wikipedia put it at 4.15 as did ChemEurope, but Gemology Project had it much lower, between 3.60-3.68, and Gemdat was lower still at between 3.25 to 3.40 – these lower measures are closer to the measures I have seen for grossular garnet. I’m not sure what accounts for these differences – gemologists may take a different approach than others. But it makes it difficult to interpret Michelle’s results. Although certainly the blue stone’s specific gravity is much lower than would be expected if it is corundum.  

For the hardness testing, Michelle scratched the blue stone with a steel knife, finding that the knife left a silver-coloured transfer mark on it but not an obvious groove. This suggests the stone is harder than the steel. Then she found that the stone left a scratch on glass. Thirdly, the stone produced a clear scratch in some crystal quartz – when the test was reversed, the quartz did not visibly scratch the stone but instead, the sharp point of the quartz became blunted/worn. So Michelle’s blue stone is reasonably hard. Hydrogrossular garnet is hard, so much so that Maori used it for hammer stones.

It’s clear from Michelle’s testing that her blue stone (Stone #1) has the highest specific gravity of the four, though the variation between the other three of 0.18 is the same as how much higher the blue stone is above Stone #2. The fact that the blue stone does not fluoresce is very suggestive that it is significantly different from the three hydrogrossular garnets. 

I am very impressed with Michelle’s testing and how she carefully recorded it. It is unfortunate that, it seems to me, there is some ambiguity about the standard specific gravity of hydrogrossular garnet, which is not helpful.

The next Post is about another Gemstone Beach fossick with Chrissy. An Index to all the Posts in this Series is here.  

I drove from Riverton to Orepuki this afternoon, arriving at Gemstone Beach at 3.30pm. I was on my way to visit Chrissy at Papatotara to see the newly arrived books “What’s This Stone?” and to discuss Thursday’s book launch. It was windy, about 14 degrees, with storm clouds in the near distance. It was a dull afternoon, not the best light conditions for looking for interesting beach stones. Two cars were leaving when I parked, and no-one else was around. There was a good patch of stones in front of the car park. They seem to have been freshly turned over, and a diversity of colour and pattern could be seen. I crossed the Taunoa Stream, noticing a recent slip in the cliff, and fossicked among a good scattering of stones along the beach to the northwest. 

A deserted Gemstone Beach, pre-storm.

The crib in the cliff.

The Taunoa Stream.

Slip on other side of Taunoa Stream.

Stones along the beach on the other side of the Taunoa Stream.

Next photo zooms in on a stone showing a reflection of the photographer.

There had been some cliff crumbling there as well, and sand had been shifted, exposing some of the orange sandstone platform that seems to underlie the beach. Some soft iron-stained stones had come out of the cliff.

Exposed platform of orange sandstone.

Patterns in the sandstone.

Cliff erosion.

Iron-stained stones from the cliffs.

I turned around after about 400 metres from the stream. Then a fierce southerly storm hit me – high winds and blinding cold rain with some hail in it. I was caught in the open. Fortunatelt I had a good rain coat on, but my jeans were drenched below my knees and water trickled into my gumboots. I struggled across the Taunoa and sheltered for a few minutes next to the crib in the cliff before making a break for the carpark, about 200 metres away. The temperature had dropped to 7 degrees, probably around 4 or 5 degrees with the wind chill. Was it worth it? Of course! Here are 11 of my finds.

The most interesting is perhaps this small dark red jasper which on close inspection seems to be a variety of poppy jasper:

Other side of stone.

Another intriguing small stone, with fuzzy-edged crystals:   

Two brown stones, maybe sugar quartz, variations of darker and lighter:    

I found a few argillite stones with trace fossils in them. This first one is packed with strongly expressed relatively large traces, with a couple of smaller and brighter ones overlying. I took it along to Thursday’s meeting of the Southland Geological and Lapidary Club and entered it into the night’s competition for recent finds.   

Trace fossils in argillite.

However, there were three or four other very interesting entries which attracted all the votes.

This second trace fossil stone has green traces:   

Argillite with trace fossils.

The third is a more common combination of colours, and contains quite a few traces, many cnsisting of lines of chevrons:    

Another argillite stone with trace fossils.

I continue to be fascinated with the variations of dark igneous stones with white crystals of different shapes, sizes and proportions. Here are four I picked up today:   

My next fossick takes me back to the Beach Beach at Riverton Aparima.    

An Index to all the Posts in this Series is here.