Gems, Minerals and Rocks Archives - North Brisbane Lapidary Club

Feldspars

May 20, 2021 By Carol

by Ray Wilson

Feldspar is the name of a large group of silicate minerals that can be found in igneous, sedimentary, or metamorphic rocks. Feldspars are commonly used in lapidary. The coloured varieties make great cabochons and the clear type that can be found at Hogarth’s Range (west of gold coast) or Springsure Qld. can be faceted. Feldspar has a hardness of 6 on the Mohs scale, so not as hard as quartz or agate.

Feldspar is an important industrial mineral being used to manufacture plate glass, container glass, ceramic tiles, plastics and other products.

Anyone interested in cabbing some Spectrolite see Carol South as the club has some material for sale.

Labradorite – a variety of plagioclase Feldspar. Named after a location in northern Canada. Labradorite displays an iridescent optical effect (or schiller) known as labradorescence. Most material now comes from Madagascar.

The cause of this optical phenomenon is lamellar structure (very thin plates or flakes) with light being diffracted.

Spectrolite – A variety of plagioclase Feldspar found only in Finland. Spectrolite is a trademarked name for this material.

The difference between Labradorite and Spectrolite is mainly that the background material in Labradorite is clear or translucent while Spectrolite the background is black. Spectrolite tends to have more vibrant colours in reds, purples, and oranges. Labradorite is generally vivid blues and yellow/bronze colours.

Moonstone – an orthoclase Feldspar that shows a pearly sheen or schiller. This is the basis of the name moonstone. Its name is derived from a visual effect, sheen or schiller (adularescence), caused by light diffraction within a micro-structure consisting of regular lamellae.

Sunstone – is a plagioclase Feldspar that has either a hematite (iron stone) or copper inclusion. Sunstone fund in Kong Bore Northern Territory has a hematite inclusion and appears as a series of rainbow-coloured specks rather than a schiller as in Labradorite or spectrolite

One variety of sunstone also found in the Northern Territory is a very rare Rainbow Lattice Sunstone

Oregon Sunstone from Oregon in USA has a copper inclusion which gives the stone a coppery coloured schiller.

Flat Lap – New Equipment Available

February 11, 2021 By Carol

Using the 150mm diamond wheels for any stone larger than about 50mm is not permitted in the club. The reason for this is that it causes excessive wear on the diamond wheels which are expensive, and on a technical level, it is not possible to get a perfectly flat surface on a curved wheel. The only way to achieve this is either manually on a piece of glass or using the flat lap.

There are several laps provided with the machine, but all grinding from coarse through to prepolish are achieved using a rotating steel disk to which an appropriate grade of silicon carbide grit is added. Water is dribbled onto the disk to make a slurry and then the rock is held firmly against the steel disk with the silicon carbide being an abrasive element to the task. As the rock face is flattened, finer and finer grades (from 80, 200, 400, 600 up to 1,200 grade) are used to get rid of all scratches and defects on the surface of the rock. Important! Between each grade of grit the steel disk and the rock need to be thoroughly washed to remove all traces of the coarser grit.

Once grinding has been completed and everything washed thoroughly, the polishing process can be commenced. For prepolish, a copper lap is provided which should be used with 3,000 diamond paste.

To polish, a leather faced lap is fitted, to which a damp slurry of tin oxide is added to this lap to complete the polishing process.

Peter has provided a user manual, with which members should be familiar before using the machine.

When next in the club, have a look at this machine, adjacent to the saw room.

Petrified Wood

September 17, 2020 By Carol

by (Woodworm) Christoph

Here are some things about petrified wood which get me excited….. well, not just wood, all permineralised plant material.

It all started in Germany where I was born in the town of Chemnitz, world famous for its Permian petrified forest and the local museum specialising in petrified wood from around the world and particularly in Permian flora.

As a child I always loved going into the Museum für Naturkunde Chemnitz. I was amazed by the fine detail of preservation right down into the cell structure. An ancient plant turned into rock. As I grew up I met a few collectors, called wood worms, and I was hooked even more. We went on field trips all over Europe and I still have a large collection over in Germany at my parents place.

Now how does petrified wood form?

Most importantly is that the wood is covered and locked away from oxygen, so it can’t rot away. In many cases petrified forests are links to volcanic activity. If things go right there is an eruption, which produces mostly ash and this ash will bury the forest. Over millions of years, minerals, predominantly quartz with other trace elements and metals will slowly impregnate the cell structure turning the wood into stone. This process is still not fully resolved.

The other way is wood gets buried by sediments such as sand. The process is the same, key is the presence of enough minerals to impregnate the wood. There have been cases that animal traces such as toredo borers (known as the famous peanut wood from the Kennedy ranges) or termites with their excrements (coprolites) have left their traces.

Australia has got a good diversity to offer from the Permian deposits in the Bowen basin to the well sought after Jurassic woods, Donpoxylon and ferns from central Queensland around Miles and Chinchilla and Lune River in Tasmania. Cretaceous peanut wood from WA and more recent tertiary opalisiert woods from Springsure. There are many more places to find it.

A good tip is the display in the Miles historical village. It houses the collection of former Norman Donpon, a mad collector which I was privileged to meet a few years ago.

Now I could go on and on about it …..

Below are a few more pieces from my collection.

Jasper

August 13, 2020 By Carol

Do you know Mohs?

July 16, 2020 By Carol

by Katarina

Mohs – Carl Friedrich Christian – was an old German fella, a geologist and mineralogist. He lived some time ago (1773 – 1839). You wonder why I mention Mohs? Well, he expanded on the knowledge from Theophrastus and Pliny the Elder who determined that Diamonds where pretty hard compared with other minerals such as Quartz.

Good old Carl used the knowledge from those who came before him and it became the basis of the hardness scale he developed, today known as Mohs’ Scale of (Mineral) Hardness.

When Mohs developed his hardness scale in 1812, very little information about mineral hardness was available. He simply selected ten minerals that varied in hardness and arbitrarily placed them on an integer scale from 1 to 10. It was a relative scale in which a mineral of unknown hardness could be tested against a group of ten index minerals to see where it positioned on the scale.

You most certainly have heard talk around the club house of how hard a piece of rock / mineral / gemstone is when people want to polish them.

This hardness is actually measured by the Mohs Scale of Hardness. The scale measures the scratch resistance of minerals, how difficult it is to scratch a specific mineral. This has an impact on how difficult it is to polish one of our pieces into a beautiful cabochon or faceted stone.

The highest resistance to scratches is found on diamonds and they have been assigned the hardness 10. The least resistance to scratches is talc – it basically scratches itself – and it therefore has the hardness of 1.

The higher a mineral is on the scale, the harder it is to polish. And the reverse works of course too, the lower the ranking, the easier it is. Which of course means you could polish a low ranking mineral away to nothing in no time.

A good indicator to how hard a mineral might be is sawing a piece. The more the saw has to work, the harder it usually is.

But all in all, the scale doesn’t mean much. After all, what does it mean for example, that an Amethyst has the hardness 7 if you have nothing to compare it with? The numbers on the scale are just that, numbers.

So I have searched the net and asked my friend Google if there are any examples for the hardness. And as Google knows everything, it has delivered. The combined findings are in the table.

One of the comments that I remember is that Stones with hardness of under 4 may not polish at all or will only take a light shine. Most of the stones hobbyists polish are found in the 6 to 7 range.

As you will see in the table, many minerals don’t have a specific hardness but rather sit on a sliding scale, depending on the composition of the minerals.

And if you ever find yourself in Vienna, why not look for the memorial plaque in honour of Carl Mohs.

Rocks ‘n Minerals – Do you know the difference?

June 5, 2020 By Carol

by Katarina Hayston

Being a member in the lapidary club means we are dealing with a lot of hard material, namely rocks. Their beauty lies in the eye of the beholder but most of them will get prettier the more you polish them.

But do you know whether you are holding a rock, a stone or a mineral?

I considered this on one of these self-isolation days and thought that there would be a simple explanation. Sure there is, but as usual the more you look the more answers you will find.

Let us start with “stone”. The general definition says that a Stone is piece of rock, made of hard compacted minerals, weathered to a smooth finish. Of course there are other definitions for stone, but none that apply to us here.

As we find both “rock” and “mineral” in the definition we can ignore “stone” from here on in as it is only a piece of what we are looking for.

As the definition of stone tells us that “rock” is made of “minerals” we can deduce that “minerals have to be first in order to form rocks.

The definition for mineral says that it is a solid, naturally occurring inorganic substance. Diving deeper we find that a mineral is a solid formation that occurs naturally in the earth.

Furthermore, a mineral has a unique chemical composition and is defined by its crystalline structure and shape (usually, although some do not) and it is formed naturally by geological processes (Thank you Wikipedia).

They have definitive chemical makeup as they are always made up of the same materials in nearly the same proportions.

The most common mineral structure is silicate as they contain the most common elements found in the Earth’s crust: Silicon and Oxygen. Other elements such as aluminium, magnesium and such may be present in small quantities. The most common mineral know is Quartz.

Minerals are also defined through their physical properties, which most often than not are:

Crystal structure: see below
Hardness: on the Mohs scale, a ten-point scale running from the softest, talc to the hardest, diamond.
Lustre: appearance in light
Colour
Streak: colour of a mineral when it has been ground to a fine powder. Often tested by rubbing the specimen on an unglazed plate.
Cleavage: how mineral splits along various planes
Fracture: how it breaks against its natural cleavage planes
Specific gravity: density compared with water
Any other properties (this one is my favourite)

Now that we have covered the basics of minerals we can turn to rocks.

Our stone definition has already told us that minerals are needed to form rocks. But in order for minerals to become rocks they have to be occurring naturally, in a solid combination.

Rocks are naturally occurring and coherent aggregate of one or more minerals.

Rocks are commonly divided into three major classes according to the processes that resulted in their formation. These classes are

(1) Igneous rocks, which have solidified from molten material called magma. Example: Basalt, Granite

(2) Sedimentary rocks, those consisting of fragments derived from pre-existing rocks or of materials precipitated from solutions. Example: Sandstone, gypsum, Opal (a siliceous sedimentary rock)

(3) Metamorphic rocks, which have been derived from either igneous or sedimentary rocks under conditions that caused changes in mineralogical composition, texture, and internal structure. Example: Slate and some marbles

These three classes, in turn, are subdivided into numerous groups and types on the basis of various factors, the most important of which are chemical, mineralogical, and textural attributes. And rocks can also contain organic matter, whereas minerals do not.

Maybe this helps to explain the mineral & rock situation:

Elements are needed to form Minerals. Depending on the quantity of each element present, the same elements may form different minerals.

One or more minerals (in this example all three minerals are needed) form rock under certain circumstances.

Amethyst

May 1, 2020 By Carol

Much of the faceted amethyst in the market is eye-clean, meaning it lacks eye-visible inclusions.

African material, especially from Zambia, can be a highly saturated raspberry colour. It tends to have more inclusions than Brazilian material. However, due to its remarkable colour, this is considered acceptable in a faceted stone. Eye-clean material of the same colour is more valuable.

Amethysts with eye-visible inclusions but attractive colour are usually cut as cabochons or fashioned into beads. Cabochons and beads that display fine colour and high clarity sell at a substantial premium.

Rarely, amethyst undergoes fracture-filling treatment of its surface-reaching fractures to improve apparent clarity.

Amethyst is cut into a variety of standard shapes and cutting styles. These include rounds, ovals, pears, emerald cuts, triangles, marquises, cushions, and others. Facet patterns include the classic triangular and kite-shaped facet arrangements called brilliant cuts, rows of concentric parallel facets called step cuts, and mixed cuts that combine both facet arrangements.

Amethyst is also a very popular gem for cutting into a variety of freeform shapes. This is done by hand or by automated cutting. These so-called fantasy or designer cuts can be mass produced or created as one-of-a-kind pieces. They sometimes display concave faceting, where normally flat facets have a concave shape. Other types of cuts and carvings are also found, including carvings of animals and other objects.

QLD – Kuridala (near Cloncurry)
Kuridala is located about 65km south of Cloncurry in North West Queensland. This is probably the most well-known location for fossickers.

QLD – Castle Mount/Amethyst Castle (near Cloncurry)
Castle Mount is about 100km south of Cloncurry in North West Queensland. Amethystine quartz is found at this area

WA – Wyloo Station
The “Great Australian Amethyst Mine” is located on Wyloo Station in WA. Permission from the owners is required before visiting this location.

NT – Central Harts Range
Excellent Amethyst crystals and sceptres can be found in the Hart’s Range.

NT – Victoria River Downs Station

NSW – Corona Station (near Broken Hill)
Very nice Amethyst has come from the Corona Station near Broken Hill.

Moses described it as a symbol of the Spirit of God in the official robes of the High Priest of the Jews, and the Russian Empress Catherine the Great sent thousands of miners into the Urals to look for it. In popular belief, the amethyst offers protection against drunkenness – for the Greek words ‘amethystos’ mean ‘not intoxicated’ in translation.

It was said to protect crops against tempests and locusts, bring good fortune in war and in the hunt, drive out evil spirits and inspire the intellect.

A little study of the works of Pliny will reveal that this gemstone, if worn round the neck on a cord made from dog’s hair, affords protection against snakebite.

Later, Hieronymus even reported that eagles placed an amethyst in their nest in order to protect their young from the self-same danger.

Apart from these powers, gemstone therapists say that the amethyst has a sobering and cleansing effect. Amethyst has also been said to quell excessive stomach acid and, according to Hildegard von Bingen, served to combat insect bites and beautify the skin.

But the amethyst not only had a firm niche in medicine; it was also esteemed as a stone of friendship. And since it was thought to put the wearer in a chaste frame of mind and symbolise trust and piety, the amethyst came to occupy a very prominent position in the ornaments of the Catholic clergy over the centuries. It was the stone of bishops and cardinals; we find it in prelates’ crosses and in the so-called Papal Ring (Italian, 15th century) in the Jewellery Museum in Pforzheim.

In ancient times, amethyst was already being engraved and cut into sculptured forms, witness the bust of Trajan which Napoleon captured in Berlin. Amethyst quartz, banded with whitish layers, is particularly good to work with, though it is only ever either translucent or opaque or somewhere in between. In earlier times, people liked to drink wine from amethyst cups, which brings us back to the stone’s protective function against alcoholism. According to the ancient Greek saga, Diana turned a nymph whom Bacchus loved into an amethyst; hence the term Bacchus stone. Anyone wishing to protect a drunkard from delirium mixed some pulverised amethyst into the person’s drink.

Synthetic (laboratory-grown) amethyst is produced by a synthesis method called hydrothermal growth, which grows the crystals inside a high-pressure autoclave.
Synthetic amethyst is made to imitate the best quality amethyst. Its chemical and physical properties are the same as that of natural amethyst and it cannot be differentiated with absolute certainty without advanced gemmological testing (which is often cost-prohibitive). One test based on “Brazil law twinning” (a form of quartz twinning where right and left hand quartz structures are combined in a single crystal[15]) can be used to identify most synthetic amethyst rather easily. It is possible to synthesize twinned amethyst, but this type is not available in large quantities in the market.[6]
Single-crystal quartz is very desirable in the industry, particularly for keeping the regular vibrations necessary for quartz movements in watches and clocks, which is where a lot of synthetic quartz is used.
Treated amethyst is produced by gamma ray, X-ray or electron beam irradiation of clear quartz (rock crystal) which has been first doped with ferric impurities. Exposure to heat partially cancels the irradiation effects and amethyst generally becomes yellow or even green. Much of the citrine, cairngorm, or yellow quartz of jewellery is said to be merely “burnt amethyst”
Wiki-how – testing amethyst
And for the more scientific minded, check out the paper on GIA website

The Four C’s