Quartz
Characteristics
- Habit - prism and pyramid
- Form - hexagonal or trigonal
- Hardness - 7 (hard)
- Cleavage - none
- Fracture - conchoidal
- Streak - white
- Lustre - vitreous
- Colour - purple, brown, colourless, white, pink
Quartz samples from the day workshop.
Introduction
The commonest mineral of silicon and oxygen (two most abundant elements in the Earth's crust) and when combined = silica. Grains of sand come from the weathering of quartz rocks. Long lasting due to resistance to physical and chemical change. When it has good transparency or found in attractive colours it is cut as a gemstone or used for carving.
Evidence of use in prehistoric times when used as objects (vases, sculpture, decorative items), beads and good luck charms. Now seen as a low cost gem material, it includes beautiful and diverse gems in a variety of form and colour. Many trade names for gems in the silica group with birthstones being Bloodstone (March) and Sardonyz (August).
Quartz family includes crystalline and cryptocrystalline varieties. Two types of crystalline quartz:
- 1) macrocrystalline which occurs as indirect, sometimes large crystals, sometimes in geodes. eg: rock crystal, amethyst and citrine. Mohs' of 7 so good for gem use.
- 2) polycrystalline, also called microcrystalline or cryptocrystalline, depending on the size of the grains. Grains don't have any external regular shape but made up of compact masses of minute crystals. Compact varieties often grouped under heading of chalcedony eg: agate and carnelian. Slightly softer but still tough.
White quartz can occur as drusy formations often as small pockets within agates and jaspers.
Small amounts of aluminium cause the dark coloration of smoky quartz when the crystal has been exposed to natural radioactivity.
The reason for the pink colour in rose quartz is not fully understood; titanium or manganese may be involved, as may minute fibrous crystals of a complex mineral within the quartz.
Milky quartz is white and cloudy as a result of tiny bubbles of fluid (liquid and/or gas).
Varieties
Amethyst
Best known gem variety of macrocrystalline quartz. Due to its abundance, quartz is affordable but the purple makes amethyst a little more expensive, esp. if with good clarity and well cut.
Colour is due to small amounts of iron being naturally irradiated from rocks over time to form a 'colour centre'. Can show weak pleochroism from mauve or blue to red-purple. can be treated to lighten the colour and transform into that of a citrine, or turn green - then called 'green amethyst' which is a misnomer as amethyst by definition is purple. Minute quantities of Fe3+ iron induce a purple coloration, characteristic of amethyst.
Synthetic is produced in large quantities and it's hard to tell the difference.
Sources include Brazil, Russia, Uruguay, USA, Bolivia, Zambia, with small occurrences elsewhere. Amethyst crystals often found lining cavities in basalt rock and form complex shaped geode which are collected to display in museums and private collections.
Quality factors: 1) highest - deep blue-ish purple, not too dark, clean stone, good cut 2) then - good purple colour, clean stone 3) then - slightly lighter purple and not as clean 4) then - pale purple but fairly clean 5) then - heavily included, even with a darker colour
Lore: February birthstone; thought to keep wearer protected from the effects of alcohol; Medieval times also to keep wearer awake and sharpen the wits, promote chastity and prevent diarrhoea.
Citrine
Yellowish-brown, macrocrystalline quartz. Weakly dichroic and may show two shade of yellow but difficult to see in pale stones.
Name is from the French for lemon (citron).
Occurs naturally but much on the market is the result of heating amethyst. Natural citrine with goo, even colour is rare. Beautiful examples from Russia, Madagascar and Brazil.
The yellow colour of citrine is probably due to minute amounts of iron hydrates dispersed within the crystal. Many examples of citrine for sale are actually artificially heated or irradiated amethyst.
Quality factors:
1) highest is strong orange-yellow, clean stone, good cut
2) then a yellow colour with large window on the stone
3) then greenish-yellow, pale colour ('lemon quartz')
4) then good orange colour but heavily included
Rock crystal
Most prominent crystalline is rock crystal, clear & composed of silica (SiO2). Usually found with good terminations, often in crystal clusters. Transparent and colourless and can have inclusions of vegetable matter. Can occur in beautifully shaped transparent crystals, possibly metres long.
In the past often used as imitation of diamond & for carved objects. Today rock crystal with inclusions of other minerals are popular eg: needle-like tourmaline or golden rutile, and is a favourite for carved skulls, crystal towers, spheres and beads.
Lore; thought to be ice that was thoroughly frozen so never thawed. Comes from Greek for 'ice'. An April birthstone.
Agate
Generally less than 15cm in diameter. Comprised of one or more varieties of quartz (quartz contains mainly silicon and oxygen) but exhibiting varieties of colour and form. Cutting in half will show how they differ in amount of quartz present as well. May be entirly of chalcedony (v. fine grained quartz / polycrystalline) or chalcedony with one or more of the following varieties: common quartz, amythest, smoky quartz, carnelian and jasper. Other common minerals may include calcite or celadonite.
Also cryptocrystalline - crystals are so tiny that no distinct crystal patterns are visible even at high magnification. Distinct colour banding, usually white to blue-white and browns. Formed when minerals in water slowly build up inside spaces in other rocks. colour comes from tiny bits of other materials that mix when the crystal is growing. Relatively porus so can be stained in any colour. Used for carvings and gems since ancient times. When binding is straight is called oynx.
Some have fluorescence - rare.Eg: Sweetwater & Youngite from Wyoming. Under UV it is not the agate that glows but trace impurities - activators - such as manganese, rare earths and uranium.
Lore: Middle Ages agate thought to calm storms, improve sight, increase strength and make the wearer eloquent and polite.
History: German village of Idar-Oberstein is a centre for gem cutting. Since 1400s gem cutters came for work, although thought the history of mining went back to the Romans, eg: agate, jasper, amethyst. even today there are multigenerational expert cutters producing fine work. From the end of the 19th cent. gems from around the world were brought here to cute. It has its own museum of gems and worked objects.
Arab Roots of Gemolgy
Some elements from 'Arab roots of gemolgy: Ahmad ibn Yusuf Al Tifaschi's Best Thoughts on the Best of Stones' (c.1240) trans. Samar Najm Abul Huda. The Scarecrow Press, London, 1998.
Amethyst
This is an iron stone but was prevented by external factor, namely, increase or decrease in the four elements.
Found in a village called Safra three days from Tabya.
Four varieties:
- trong pink and blue colours, the best one
- strong pink but weak blue colour
- strong blue and weak pink
- weak pink and blue, the cheapest kind
Amethyst is a stone that the Arabs appreciated and used for decorating utensils and weapons. The way to cut it is the same as for emerald - ground with emery on a bed of lead with water, then polished.
It encouraged the wearer in times of battle (according to the Arabs); whoever makes a drinking glass of amethyst and drinks as much wine as he wishes, he will not get drunk; protects its wearer from gout and if placed under a pillow protects from nightmares
Rock crystal
Pliny said it is a borax-based stone, its colour being white due to the factors that affected it.
Some crystal is formed in Djazirat al 'Arab in Hidjaz, this being the best. Another variety comes from China, inferior to the Arabian one. Also comes from European countries which is good quality. Mines near Armenia which yield rock of yellowish tinge - the 'glassy' type as if cooked in fire. New mines have been discovered near Marrakesh yielding clear but cracked crystal, it became so abundant that the King of Morrocco used rock crystal for decorating the walls of a large Madjlis.
The best is the clearest, most limpid and transparent of strongest white colour, free from cracks. It melts like glass and can be easily dyed. If placed in fire it disintergrates and calcinates. If sun rays are captured within it, and a soft rag positioned here, one sees the refraction of the light coming out of the stone, the rag burns and fire can be obtained from it. (Refers to use of rock crystal as magnifiers or for cauterization of wounds. In the West the theoretical discovery of magnifying lenses was made in Italy in the 14th century.)
Used for carvings in Europe and Africa, for utensils and vessels for liquor (in the shape of a rooster). Water containers are made from rock crystal in Kashgar and in Ghaznah.
If worn, a person will not see nightmares.
The value differs according to the size and the utensils made from it. The pleasure in it is derived from the large size of the items made from it.
