Minerals are naturally occurring inorganic solids with characteristic chemical compositions and usually an ordered arrangement of atoms that forms crystals. Minerals are the substances that make up the rocks of the Earth’s crust. Scientists have identified almost 5,000 minerals, but most of these are rare and look insignificant.
Just around twenty common minerals make up most of the Earth’s crust, with most composed of just a few of the most common of the 90 naturally occurring elements on Earth.
Occasionally some minerals form crystals large enough to be seen. These crystals occur in many different shapes and colours and they can form beautiful and attractive objects.
The Ulster Museum’s mineral collections include specimens collected by the museum’s curators, items donated by members of the public or by mining organisations, and others purchased from mineral dealers or at auction.
Tungsten is a heavy, grey metal with a very high melting point giving it many roles in technology, including as the filament in electric light bulbs.
The town of Pasto Bueno, in Ancash Department, Peru is a centre of tungsten mining. The metal is found in the quartz veins of an igneous rock mass, occurring as the mineral huebnerite (manganese tungstate).
Here and there in the veins, where there is a cavity, crystallisation has occurred, producing quartz and huebnerite crystal groups. The black huebnerite and transparent quartz crystals form striking sculptural pieces.
The museum purchased this Pasto Bueno huebnerite and quartz group in June 1977 from an Englishman Mr M.W.J. Townsend. It was one of a consignment of mineral samples that he had bought some months earlier from street traders in the Peruvian mining towns.
Gold has been treasured as a precious metal since earliest times. Its attraction comes from its rich yellow metallic lustre, resistance to rust, and a softness which allows it to be easily worked by goldsmiths.
The formation of gold deposits, and their occurrence, is complicated and successful gold prospecting requires specialist knowledge.
Gold usually occurs as irregular flakes and nuggets but occasionally it may form crystals.
This fragment of conglomerate rock contains an octahedral crystal of gold. This is a rare occurrence and this gold specimen is worth more in its crystal shape than if it were melted down for bullion. The Ulster Museum bought this gold sample from Sotheby’s, London, in 1968. The only information known about it was that it was from Brazil.
In 1976 a gold prospector, Mr James Stewart called into the museum to examine this specimen. Through his expert knowledge he identified that it came from the Oro Preto goldfield in Brazil. This is the oldest goldfield in Brazil. Its name comes from the Portuguese for ‘black gold’, because the first gold nuggets found there were stained black with iron oxide.
Wulfenite and mimetite
Almost 5,000 minerals are known to scientists. Most are rare and are found sparingly at only a few places in the world. In fact, most of the Earth’s crust is made up of just eight chemical elements, which combine together to form about twenty common rock-forming minerals. Many of the minerals on show in the museum’s Earth’s Treasures display are rare, and are seldom encountered by geologists in the course of their work.
Wulfenite is lead molybdate; mimetite is lead arsenate. Crystal groups such as this have to be handled carefully because the thin wulfenite plates are fragile and the minerals themselves are toxic.
This piece was bought from the Mineral gallery, London in 1975.
Cavansite was once one of the world’s rarest minerals. For many years it was known from only one place, Lake Owyhee State Park, in Oregon, U.S.A., where it was discovered in 1967 as small blue specks in the local rock.
In 1988 large cavansite specimens were discovered in the Whagoli basalt quarry, in Poona, India. They occur as sprays of blue crystals set amongst pale-coloured zeolite minerals. Whagoli cavansite has become popular with mineral collectors, and this is now the main source of the mineral in museum collections. The museum bought this piece from an American dealer in 2009.
The mineral’s name ‘cavansite’ comes from its chemical composition: calcium vanadium silicate.
Agate is a microcrystalline variety of silica (silicon dioxide), characterised by fineness of grain and colour banding. It is found in volcanic rocks worldwide. It is a hard mineral and it has been used in jewellery since earliest times.
There are many colour and pattern varieties of agate and there is a bewildering array of names used to describe them. When some agates are cut, the angle and position of the cutting slices through the internal banding, exposing these structures as ‘eyes’.
Canon John Grainger (1830-91) a Church of Ireland Rector at Broughshane, County Antrim assembled a huge collection of 60,000 natural history and archaeological objects in his rectory. In 1891, shortly before his death, he donated it to Belfast Corporation. It was one of the founding collections of the Ulster Museum.
This piece of polished Eye Agate is from the ‘Grainger Collection’. Canon Grainger affixed this verse from Holy Scripture (Zechariah, Chapter 9, verse 9) to the back of the specimen: "For behold the stone that I laid before Joshua; upon one stone shall be seven eyes."
Pyrite is iron sulphide and it is commonly found with other ore minerals, such as galena and sphalerite. It is the most abundant sulphide mineral in rocks of all ages, and found in all three of the major rock types — igneous, sedimentary and metamorphic.
The brassy colour and metallic sheen of pyrite leads to it being mistaken for gold, giving rise to its popular name ‘Fool’s Gold’. Real gold can be identified by its softness and much greater density.
Pyrite can form beautiful clusters of crystals in a variety of shapes. Since the 1970s Peru has produced some of the finest pyrite specimens in the world. They came from mines in the Peruvian mountains and many were sold from markets set up in the centre of Lima. The high quality of the pyrites attracted international dealers and soon Peruvian pyrite became available world-wide.
The museum bought this specimen in 1975 from an English traveller Mr M.W.J. Townsend, who had bought the piece in Peru.
Quartz is the crystalline variety of silica (silicon dioxide). It is the second most abundant mineral in the Earth’s crust, after feldspar. It occurs worldwide in many rock types and in many environments.
Rose Quartz is the least common of the quartz colour varieties. It rarely forms crystals occurring mainly as shapeless veins in rock. It is not much used in jewellery, because of its cloudy appearance. However, it is a popular collector’s item because of its attractive rose tints. Some people believe that it can improve the skin, heal women’s complaints and cure broken hearts.
This Rose Quartz crystal group is from Governador Valadares, Minas Gerais, Brazil. The city is the centre of a rich gem-producing area, exporting gem minerals and finished gemstones around the world. This piece was bought from the Hatton Garden dealers ‘Roughgems’ in 1978.
There are different varieties of quartz based upon colour:
- Citrine (yellow/brown)
- Amethyst; (purple)
- Rock Crystal (clear, flawless)
- Smoky Quartz (black)
- Milky Quartz (white)
- Rose Quartz (pink)
A gemstone is a naturally occurring mineral that has been artificially polished, faceted and shaped for decorative purposes. The mineral must be hard enough to withstand wear and tear. The most sought after gemstones are clear, attractively coloured and free from flaws.
Gemstones are classified as precious (e.g. diamond, ruby and emerald) or semi-precious (e.g. garnet, zircon and topaz).
A semi-precious stone that displays a dazzling and wide variety of colours is tourmaline. It has red, pink, violet, blue, green, brown, black and colourless varieties. Some crystals display two or three colour bands along their length in shades of green and red.
The red tourmalines are called ‘Rubellite’, from the Latin rubellus meaning ‘reddish’
This specimen is from the Julius Hanna Collection of minerals. It was bought by the museum from his widow in 1926. Nothing is known about Mr Hanna, except that he was gentleman of means, who lived in Strandtown, in east Belfast.
Colour can be an unreliable guide to mineral identification. Many minerals occur in different colours, and the same colour can be seen in different minerals.
However, there is no mistaking the intense orangey-red colour of the mineral crocoite. Found in lead mines, crocoite, which is lead chromate, is also identified by its slender elongated crystals.
This crocoite specimen was donated to the museum, by a Belfast man, Mr Gray in 1932.
Gypsum, which is calcium sulphate, is a common mineral found at many localities around the world. It is far too soft to be used in jewellery but is sought after by mineral collectors, because it can form beautiful crystals, in varied and sometimes fantastic shapes.
In Victoria, Australia, a gypsum vein has produced arrangements of bladed crystals topped by sprays of needle-shaped crystals, forming a vision of a mineral flower bed.
This specimen, which is very fragile, required special packing for its safe carriage. It was bought from mineral dealer Max Davis of Oxford Street, London in 1975.