Small hand-cobbed chunks ready for beads, cabochons, or other jewellery applications with each piece being approximately 3 × 4 cm at most.
Above is a classic piece of Blue Lace showing the most common width of the vein in the deposit.
15 × 3 cm
A polished cross-section showing (from top): unidentified positive points of chalcedony, multiple bands of blue lace agate lying on green dolomite, with dolerite (black) to left, in amongst the clay, and more green dolomite below. The section is 8 cm thick.
This is another unusual piece, probably from the earlier days of the mine. It is a real mixture of blue lace, green dolomite, gingery-brown smectite or clay (a weathering product of basaltic rocks), and has green chalcedony growing in the main vug. (A vug is a cavity in a rock that may contain crystalline minerals). Its size is 20 cm wide and 15 cm high. Two close-up views of the same stone are shown.
The “Prickly Cloud” is a beautiful old piece, probably from a large vug that was exposed in the earlier days of mining. This cleaned up completely, leaving no trace of clay or calcite on its surface. It was too beautiful to work further. Size 30 × 8 cm
This shows George Swanson and a miner standing alongside a rare vug where Blue Lace formed in botryoidal druses. The picture would have been taken before 2001, when the article this photograph appeared in was written. The “Prickly Cloud” no doubt came from a vug like this.
It seems as if there are two different kinds of crystal formations that we have yet to fully understand. The first on the left shows chalcedony forming positive calcite-like points. Are they maybe after fluorite, but if they are how come there are no square or cubed crystal shapes? The other is these negative images; these apparently are calcite related.
See the picture below:
Here there are possibly still traces of the dissolved mineral visible. The specimen is 10 × 1 cm
One explanation for these areas of seemingly negative calcite crystals could be that calcite was lining the sides of the crack in the dolerite sill and the silica (chalcedony) bearing fluid flowed into the cavity pushing up against it. When more hydrothermal fluids again flowed into the crack some could have dissolved away the existing calcite leaving the imprint of the original crystals in the chalcedony.
An example of this dissolution is shown in the specimen below that I etched with dilute hydrochloric acid.
It shows examples of smooth, lacy and botryoidal chalcedony. Nearest the camera the green dolomite has been etched away and negative calcite points remain. 13 × 5 cm
Each about 4 cm long and 3 cm high.
The points on the pieces above are chalcedony pseudomorphs after an as yet unconfirmed mineral. (A pseudomorph is a secondary mineral that has replaced an earlier mineral, but has retained the original mineral’s shape). This particular combination is rare. This is older material which comes from a small pocket found in the 1980s at the south end of the mine in the crazy lace area. Chalcedony provided the silica for the replacement of possibly melanophlogite crystals that were originally on this matrix. It appears with these two particular specimens that any residual dolomite has been etched away after being soaked in hydrochloric acid by their previous owner. These specimens are another clue to the geology of the Blue Lace mine.
Below is another piece of the same material belonging to a friend who sent me the photograph. He only partially removed the original white coating, leaving evidence of what the specimen was like when first found. Recently I have located an article saying that this could be chalcedony after melanophlogite, not fluorite. (Marek Chorazewicz: Bulletin of the Mineralogical Society of Southern California. Volume 87 Number 8 – August, 2014).
Another twist in the tale…..
And in 2019, yet another mineral, halite, came into the equation…
Does this pseudomorph help explain the shapes that cover a specimen like the carving of Whopper below?
Whopper is another example of these unusual positive crystal shapes on domed intrusions formed by the chalcedony in wider cracks of the dolerite. In the right hand photo are two other examples showing what is often on the inside of many of these rounded pieces.
This was an unworked piece of rock awaiting possible carving. It has now become Nibbly, the polar bear – see the Gallery. It clearly shows the multiple layers of chalcedony that have possibly grown over the central dolerite and dolomite, ending up with the positive points on the outermost layer. 19 × 10 cm
This piece I call the “Ooze” because it may well be showing how hot, silica-rich fluids made their way over time into existing cavities in the dolerite sill, and microcrystalline chalcedony formed on the outer walls, eventually almost completely filling some cavities, and sometimes precipitating quartz crystals in the core. Two close-ups of the same stone are below. The whole stone is 28 × 12 cm.
Here is another example of how silica-rich fluids were deposited in narrow layers on the walls of green dolomite. In the centre a wider band of band of chalcedony has formed, and later been overgrown by drusy quartz. At the top left of the stone are some positive points of chalcedony and a small overlay of drusy quartz on top of that. I have worked off the green dolomite along the sides, as this is a display piece.
Below is a polished section of the back of the same stone. 30 × 12 cm
This is the lacy effect that the layers of chalcedony can create. It is from the carving I called Bridget Lacey. No doubt this effect gave Blue Lace its name. Below is a close up of the “lace”.
Here is another close up of a different kind of patterning.
Sometimes clear quartz crystals will fill a wider vug. 126 × 13 cm
Here Blue Lace is lined with large quartz crystals and then in the centre there is more chalcedony and finally a drusy quartz crystal-lined vug. This is a top view of Oscar the Owl’s head. 18 cm wide.
This is another very large piece showing sparkling botryoidal druses; its colour is a good strong blue. The intensity of the blue can vary at any level in the mine. Weight 21 kg and height 35 cm
Even newer is this piece. It was mined at 50m depth in 2016. It is again a good strong blue, and shows incredible contortions
in the flow of silica rich fluids. It weighs 2 kg and is 150 × 150 ×150 mm.
On the neighbouring farm Middelpos, which lies to the west of Ysterputs, there are stringers of blue chalcedony that run through the farm, probably related to the main Blue Lace deposit. Despite having been extensively prospected, nothing major was found. Small veins producing layered chalcedony suitable for making beads and other small items were discovered, but soon exhausted. The specimen above comes from Middelpos, but does not show the same layering. 23 × 8 cm
Here are examples of striped chalcedony from Malawi (at back) and from Turkey in the foreground.
This is blue chalcedony from farms Otjoruharui & Troye, Namibia.
And a different kind of blue and white agate from Grootberg, Namibia.
