A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.
Halley's Comet is the only known short-period comet that is consistently visible to the naked eye from Earth, appearing every 72–80 years. It last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061. Officially designated 1P/Halley, it is also commonly called Comet Halley, or sometimes simply Halley.
Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.
Lightning Ridge is a small outback town in north-western New South Wales, Australia. Part of Walgett Shire, Lightning Ridge is situated near the southern border of Queensland, about 6 km (4 mi) east of the Castlereagh Highway. The Lightning Ridge area is a centre of the mining of black opal and other opal gemstones.
The nucleus is the solid, central part of a comet, formerly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublime and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust.
Fulgurotherium is a dubious genus of ornithischian dinosaur from the Late Cretaceous (Cenomanian) Griman Creek Formation. It lived in what is now Australia.
The Andamooka Opal is a famous opal which was presented to Queen Elizabeth II in 1954 on the occasion of her first visit to South Australia. The opal was mined in Andamooka in 1949. The opal was cut and polished by John Altmann to a weight of 203 carats (40.6 g). It displays a magnificent array of reds, blues, and greens and was set with diamonds into an 18 karat (75%) palladium necklet.
Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral contained in nacre, with a microstructure inherited from the shell. It is one of few biogenic gemstones; others include amber and pearl. In 1981, ammolite was given official gemstone status by the World Jewellery Confederation (CIBJO), the same year commercial mining of ammolite began. It was designated the official gemstone of Lethbridge, Alberta, Canada in 2007, and was subsequently designated as Alberta's official gemstone in April 2022.
The Flame Queen Opal is perhaps the best-known example of "eye-of-opal", an eye-like effect created when opal in-fills a cavity.
Deep Blue Sea is a hat designed by Australian milliner, Ann-Maree Willett, which is made from Australian wool felt and set with 26 precious opals from Lightning Ridge, weighing a total of 1447.5 carats.
The Galaxy Opal is the world's largest polished opal, certified by the Guinness Book of World Records in 1992. It was found at the Boi Morto Mine in Brazil in 1976. The finished opal weighs approximately 3,749 carats, or 0.75 kg. It was carved from a grapefruit-sized piece of rough stone weighing 5,205 carats by Scott Cooley into a shape resembling a child's head. The weight of the finished stone was beyond the capacity of any gem measuring scale at the Gemological Institute of America and an approximation of its weight in carats had to be determined using a postal scale. The stone measures 14.0 by 10.2 by 4.1 centimetres. It is currently part of a private collection.
C/2001 OG108 (LONEOS) is a Halley-type comet with an orbital period of 48.51 years. It was discovered on 28 July 2001 by the LONEOS telescope at Lowell Observatory. Of the short-period comets with known diameters and perihelion inside the orbit of Earth, C/2001 OG108 is the second largest after Comet Swift–Tuttle.
The Olympic Australis opal is the largest and most valuable opal yet found and was valued at A$2,500,000 in 1997. It was found in 1956 at the 'Eight Mile' opal field near the town of Coober Pedy in South Australia.
Comet Swift–Tuttle is a large periodic comet with a 1995 (osculating) orbital period of 133 years that is in a 1:11 orbital resonance with Jupiter. It fits the classical definition of a Halley-type comet, which has an orbital period between 20 and 200 years. The comet was independently discovered by Lewis Swift on July 16, 1862 and by Horace Parnell Tuttle on July 19, 1862.
EdmondHalley was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720.
A great comet is a comet that becomes exceptionally bright. There is no official definition; often the term is attached to comets such as Halley's Comet, which during certain appearances are bright enough to be noticed by casual observers who are not looking for them, and become well known outside the astronomical community. Great comets appear at irregular, unpredictable intervals, on average about once per decade. Although comets are officially named after their discoverers, great comets are sometimes also referred to by the year in which they appeared great, using the formulation "The Great Comet of ...", followed by the year.
The Star of Adam is an oval-shaped blue star sapphire, currently the largest star sapphire in the world. It weighs 1,404.49 carats. Prior to its discovery in 2015, the Black Star of Queensland, weighing 733 carats (146.6 g), was the largest star sapphire gem in the world.
Weewarrasaurus is a genus of ornithopod dinosaur from the Late Cretaceous of the Griman Creek Formation near Lightning Ridge, in New South Wales, Australia. The type and only species is W. pobeni, known from the holotype, an isolated dentary preserved in opal, as well as a secondary referred dentary. It is thought to have co-existed with multiple other ornithopods of different sizes and lineages.
Halley's Comet is a short-period comet visible from Earth every 75–76 years.
