Stories surrounding the rocks

 kw: book reviews, nonfiction, rocks, minerals, stories, legends, surveys

This is a shelf of my own Lapidarium (rock collection), chosen for its variety. If you click on the image you can see a larger version.

As a rock collector I have eclectic tastes. Here are minerals, fossils, sliced-and-polished geodes, and a few crystal clusters that were grown in a jar (such as the deep blue at bottom center: copper sulfate, which occurs in nature in very arid regions as the water-soluble mineral chalcanthite).

The word "lapidarium" is quite obsolete, but is found, very fittingly, in the title of a recent book, Lapidarium: The Secret Lives of Stones by Hettie Judah. Ms Judah writes primarily about art, particularly the stories surrounding artworks; here, the artworks are natural stones and modified stones.

Sixty stones, loosely so-called, are surveyed in six sections, with the themes Power, Sacredness, Evocativeness, Technology, Sculpture, and Life. Life? Consider Coral. In a game of Twenty Questions, the answer to the obligatory opening question, "Animal, vegetable, or mineral?" is "Yes" or "All". The polyp animal, bearing symbiotic algae in its tissues, secretes a mineral skeleton. The deep red skeleton of precious coral, so sought after by kings of old, is colored by pigments derived from the algae. You can only get away with this once.  The Life section also mentions Pearl and Blue Lias; the latter is a heavily fossiliferous shale on England's Jurassic Coast. A few examples from the other sections:

Power: Powerful people like gemstones such as emerald, ruby and sapphire. Note that ruby and sapphire are both corundum (aluminum oxide), with different minor impurities that confer their colors. So this is clearly not a mineralogy book. I firstly thought that Old Red Sandstone was an odd choice; it turns out that the famed Stone of Scone, where Scottish kings were anointed, is a big block of Old Red Sandstone.

Sacredness: Pele's Hair, a type of spun glass from volcanic lava fountains, is sacred to the Hawaiians. Sarsen is silicified sandstone (much tougher than the usual calcite-cemented sort) that makes up the larger stones at Stonehenge. Cinnabar, or mercury sulfide, is a bright red mineral that was used to make scarlet pigment for cosmetics and the clothing for royals and ecclesiastical officers (and poisoned a number of them). Tuff was used to form the Moai, the standing sculptures on Rapa Nui (AKA Easter Island). I must mention that, while the article mentions a study in 1997 of moving one of the statues using ropes, a sledge, and banana-juice lubricant, the moai actually "walked". See this 2013 video. It is a clip from a longer presentation.

Evocativeness (the section is called Stones and Stories): Lapis Lazuli, translated "azure-colored stone", was also a stone of power, but later became better known as the source of Ultramarine pigment for the intense blue and indigo colors in paintings of the great masters. Phonolite Porphyry is a "ringing stone" that makes up the substance of Devil's Tower in Wyoming. Tap a piece with a smaller stone and it will ring like a gong or bell. So will most of the Dolerite stones that make up the inner circle at Stonehenge, and there is a field of ringing stones in eastern Pennsylvania.

Sculpture: Gypsum (usually as alabaster), chalcedony, onyx, and crystalline quartz are among the stones favored for carving and sculpting into artworks. Red Ocher, a mix of red hematite and other iron oxides, may have been used for art tens of thousands of years ago, both as a pigment and as a material to sculpt.

Technology: Flint may be the basis of the oldest technology, that of making edged tools of stone by knapping; the article is mostly about a modern flint knapper who became so skilled, many museums have his counterfeits on display. Obsidian may be equally old, but its occurrence is not as widespread. Lodestone, magnetic ferrous (Iron II) oxide, appears to have a nearly 1000-year history as an aid to navigation. Coltan, an ore containing niobium (columbium) and tantalum, is still in heavy use for electronics. Mica, particularly the clear, colorless form called muscovite (named for Moscow), was used for oven door windows because it is heat resistant; in such a use it was called Isinglass.

This is a fun, well-written book, not about rocks per se, but about stories about rocks.

This spring's stony harvest

kw: rocks, rock collecting, jasper, lapidary, photographs

A couple days ago I cleaned up the latest batch of rocks from my tumbler. These 32 stones are all Lavic Jasper that I collected in 2008. Their finished weight ranges from 88 grams down to just under 5 grams. I made closeups of five that I find particularly attractive.


This one looks the most like an agate, in close-up. Lavic Jasper is known and famed for having small fortification agates embedded in the matrix. This piece has a much larger fortification.


Here we have a breccia, that seems to have broken up when in a soft state, then re-cemented. The little bluish spots are some of the agate inclusions mentioned above.


This one has more of the blue agate showing. This feathery matrix is called plume jasper.


Much of the Lavic Jasper is dark brown, which is usually uninteresting. This piece, however, has a banded and swirly appearance that reminds me of the planet Jupiter's cloud bands. The piece is just 1.5x2 cm.


Here we have another piece that has a larger agate section. You can actually see into the stone in the dark vein across the middle.


All this variability shows why Lavic Jasper is my favorite semiprecious gemstone.

It is mostly what holds you up

kw: rocks, mineralogy, gemstones

My screen saver is a picture gallery program, and at present it is pointed at images of mineral crystals. Some I took, mostly at museums but a few of my own specimens. Others were gathered from all over the internet. I have the timer set to three minutes, so the pictures frequently start up when I'm getting a little think time at the keyboard.

After lunch today, seeing the minerals, I got thinking about feldspar. Most people seldom or never heard the word, yet feldspar minerals are the most common on Earth. Maybe you know that most tombstones that aren't made of marble are of granite, either whitish or reddish. The light-colored mineral that makes up most of the composition of granite is feldspar. In granite, the white or pink feldspar is usually a type of orthoclase.

Feldspar is a kind of junkyard mineral, because it has such a variable composition, variable chemistry. There are two "solid solution series" that make up the main feldspar family of minerals. Sodium feldspar, or Albite, is one end member of both series. The composition of pure albite would be NaAlSi₃O₈. The other end member of the alkali feldspar series is Orthoclase or potassium feldspar. The composition of pure orthoclase would be KAlSi₃O₈. In this series, most orthoclase contains a little sodium, and most albite contains a little potassium, but mixes near the middle are very rare. More common is orthoclase in which a tiny bit of iron substitutes for aluminum, making it pinkish.

A whitish or yellowish orthoclase that cleaves exceptionally well is also called microcline, so named because its two cleavages are very nearly at 90°, just a tiny angle (a micro-cline) shy of a perfect right angle. If the microcline contains lead instead of iron, it can be a lovely turquoise color, and is called Amazonite:

I have sometimes wondered why Amazon.com doesn't adopt this mineral as a mascot (Image from Wikimedia Commons). Amazonite is sometimes polished into cabochon gemstones, though it is a bit fragile. Also, its hardness is equal to that of glass, so it scratches rather easily, because sand and silt contain quartz which is one step harder (7 versus 6).

Microcline, usually of the white variety, tends to crystallize in large crystals. A very coarse-grained granitic rock called pegmatite is often mostly microcline, with the other nice mineral specimens in between. The largest known crystal on Earth, a microcline crystal in a miles-long pegmatite in Russia, has been mined for road-grading gravel for decades – that one crystal! It isn't in pretty condition, and wasn't recognized as a single crystal until about thirty years ago.

There is one other feldspar stone that is sometimes made into gems, Labradorite. It is a member of the Plagioclase feldspar series. One end member is albite, and the other is Anorthite, or calcium feldspar. The composition of pure anorthite would be CaAl₂Si₂O₈. This series mixes readily enough that there are several mineral names for intermediate compositions. Labradorite is right in the middle, 30-50% albite and 50-70% anorthite. By the way, that ancient moon rock that was dated near 4.5 billion years old was nearly pure anorthite. A labradorite that is 60% anorthite would have the chemical composition Ca_0.6Na_0.4Al_1.8Si_2.4O₈.

This labradorite specimen has been polished. It reflects light with this bluish hue in a way similar to opal, because of a series of tiny offsets in the crystal structure that cause constructive interference for blue light. (Image from Wikimedia Commons.)

Labradorite is sometimes called Moonstone, but there are other minerals also called moonstone when polished. It is as fragile as polished microcline, but with care it makes a lovely gem. I have a tie pin with a labradorite cabochon, about 12x16mm. It was polished with the "blue angle" set a little upwards so it glows when the light comes from above. At other angles it looks black.

Most of the time, our contact with feldspar is by walking on it, almost any time there is a gravel driveway or walkway. These special varieties show a pretty side of this most common mineral family that makes up 60% of the crust of the Earth.

What I would cut if I were cutting

kw: rocks, minerals, lapidary

It has been a couple decades since I did any stone cutting. During the few years that I was a lapidary hobbyist, I soon found that I got the most satisfaction from cutting cabochons from precious opal.

Common opal is a whitish variety of hydrated silica (quartz plus water) with a very smooth texture. Precious opal, or fire opal, blazes with any or all colors of the spectrum. The difference is internal structure.

All opal is composed of spheres of silica, usually smaller than a micron in size, with a combination of amorphous silica and water in the spaces between the spheres. Two characteristics make common opal "common". Firstly, the spheres are larger and have a range of sizes. By "larger" I mean large enough that, no matter how they are packed, they will not diffract light efficiently; this typically means larger than one micron in average diameter. Secondly, the spheres, particularly because they are not uniform in size, are packed in a rather amorphous of chaotic arrangement. For this reason, common opal is rather soft, typically about 5.5 on the Moh's scale.

If the spheres are more uniform in size, they are likely to pack well into a closest-packing arrangement (like a well-stacked pyramid of oranges at the market). Their average size will then determine how much of the spectrum they diffract into the blazing colors of fire opal. Larger spheres diffract mainly red and yellow light, making red or orange fire opal. Smaller spheres diffract more green and blue light, while still diffracting some of the longer (redder) wavelengths. The smallest spheres produce mainly blue-green and green appearing stones.

When I was cutting opals, I found that they varied in durability. Not so much due to varying hardness, but because of how well the water content was bound in the stone. I learned to ask the dealer how stable the material was. An honest dealer will tell you, and point out which stones have to be kept in water to retain their fire, and which can be cut into a cabochon that will last the longest. Of course, the latter will cost more. Like a lot of rockhounds, I wound up with a jar of water containing nice looking opal rough sitting on my mantel, stones I'll never be able to cut, but they do look pretty.

During those years, I got to know one man who didn't own any lapidary equipment, but was cutting fine stones that won awards at rock shows. He said he'd buy a likely-looking piece of rough and grind it on the sidewalk next to his front step. If the fiery layer was buried too deep in the "common" portion of the stone, he'd pay the dealer to cut some of it off first with a diamond saw. At home he would simply hold the stone by hand and grind it on the wet pavement. He had one section of the pavement that was well worn from years of grinding, which was now his "fine grind" section. A light touch helped also, to achieve a nearly smooth finish. When he was satisfied, he'd go back to the dealer and pay to have the stone polished on a rouge-loaded felt wheel. The cost wasn't much, because a good polish took just a few minutes to produce.

Even the hardest opal is no harder than Moh's 6.5, so it cuts a lot easier than agate or jasper. I found I could make a finished opal cabochon in a quarter of the time it took me to finish a jasper "cab". If I ever return to the lapidary hobby, rather than collecting just those pre-finished items called "mineral specimens", opal will be first on my list of materials to cut.

Favorite fortifications - agates that is

kw: rocks, minerals, agates, chalcedony

I was going through some rocks recently, getting a load ready for my tumbler. I've been mostly tumbling jasper that I got at Lavic, California (I posted about Lavic Siding and its wonderful Jasper in March, 2008). But I also have a bit of this and that, different stones of different types, that I've acquired over the years. One thing I am very short of is fortification agate. I made up my mind to acquire some. There is a rock show in Delaware in March, where I'll look for pieces to buy. I'd love to get a piece or two like this one:


This is a Fortification Agate. This sort of pattern is the classic Agate. Some old Aggie marbles were carved from well-formed agates and show patterns like this. This image is a clip, a bit color-brightened, from a photo at Superagates, of a Fairburn Agate, the original Agate. All other fortification agates are compared to Fairburns, with good reason. They're the best.

There are many kinds of fortification agates, for they occur all over the world. Some have larger, coarser patterns, and some are smaller and clearer (the rock above is about two inches, or 5 cm, across). Some are more or less distinct or colorful. I do have a couple of rather insipid agates from the Laguna locality, but I'd hesitate to put up their images; almost a waste of the space.

Agates are nearly pure quartz; the impurities are mineral salts that provide the color. Nearly every color is possible, but earth tones and red-orange are most common. The quartz in an agate is crystalline, not glassy, even though they look like stained glass. They are cryptocrystalline, which just means that the crystals are so small they are hidden from our eyes. The two main types of cryptocrystalline quartz are flint and chalcedony.

In flint, the crystals are blocky and don't have any particular orientation. In chalcedony, the crystals are filamentous, like short fibers. In some forms, the fibers are not oriented in any particular way; they are like felt. Most jasper is composed of felted fibers. When the fibers are oriented, we get agates.

In fortification agates in particular, the fibrous crystals are oriented radially; the bands are formed of crystals that run the short way. In some cases, the crystals are very transparent and when they are just the right range of size, they diffract light. Then thin (2-3 mm), polished slices of the agate show iridescence, and this special kind of fortification agate is called Iris Agate. An example is shown here, in an image from the Flickr account of Rockman836:


The colors don't come from pigments, they come from the light diffracted from the oriented fibers, and the color shift results from the changing orientation of the fibers with respect to the light source. On my monitor this image is very nearly life size, taking for scale the binder clip holding the agate piece. That's quite a large chunk of iris agate. Most of what I have seen has been smaller (of course, I may be mistaking the size of binder clip that was used).

By the way, one would never tumble an iris agate. They need to be cut thin and polished on a flat lap. Other fortification agates can be tumbled, and I have seen rather large agates that were tumbled so their patterns show from all surfaces. Awesome!

Need you ask why I don't go collect my own agates? Compared to jasper, good fortification agate is quite scarce. Many of the classic collecting localities are nearly worked out, and some have become private preserves. If you look at the prices on e-Bay, you'll find that agate isn't really a "semi-precious" stone any more. At current prices, it is as costly as Jade. But buying some will cost me less than a collecting trip, so I'll celebrate oncoming old age by "silver picking" my next acquisitions.