Pearl

According to Rebbenu Bachya, the word "Yahalom" in the verse Exodus 28:18 means "Pearl" and was the stone on the Ephod representing the tribe of Zebulun. In astrology Polaris is one of the Behenian fixed stars and has the symbol . Necklaces can also be classified as uniform, where all the pearls are the same size, graduated, where the pearls are arranged in size from large in the centre to smaller at the ends, or tin cup, where pearls are generally the same size, but separated by lengths of chain. However, the bright Southern Cross (Crux) points towards the south celestial pole. An opera will be long enough to reach the breastbone or sternum of the wearer, and longer still, a pearl rope is any length that falls down further than an opera. The star visible to the naked eye that is closest to the south celestial pole is the dim Sigma Octantis, sometimes called Polaris Australis. A matinee of pearls falls just above the breasts. There is no real southern pole star.

The size called a princess comes down to or just below the collarbone. The astronomer Edward Guinan considers this to be a remarkable rate of change and is on record as saying that "If they are real, these changes are 100 times larger than [those] predicted by current theories of stellar evolution". Pearl chokers nestle just at the base of the neck. Recent research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it. A collar will sit directly against the throat and not hang down the neck at all, they are often made up of multiple strands of pearls. The star is also about 15% brighter (on average) than it was in 1900; the period has also lengthened by about 8 seconds each year since then. While most other necklaces are simply referred to by their physical measurement, strings of pearls have their own set of names that characterize the pearls based on where they hang when worn around the neck. As of 2005, the variations are about 2% from peak to trough.

There is also a unique way of naming pearl necklaces. Around 1900, the star varied between being 8% brighter than its average luminosity and 8% dimmer (0.15 magnitudes in total) with a 3.97 day period. Although imitation pearls look the part, they do not have the same weight or smoothness as real pearls, and their luster will also dim greatly. The main star is a Population II cepheid variable, the pulsations of which cause it to cycle steadily. Some imitation pearls are simply made of mother-of-pearl, coral or conch, while others are made from glass and are coated with a solution containing fish scales called essence d'Orient. It is an F7 supergiant (Ib) or bright giant (II), with two smaller companions: an F3 V main sequence star about 2700 AU away and a close companion about 20 AU distant. Imitation pearls are much easier to identify by jewellers. Polaris is 431 light years (132 parsecs) from Earth, according to measurements made by the Hipparcos satellite.

If the centre is not perfectly round, the jeweller recognises that it is genuine, and gives it a higher value. See List of brightest stars. This is because when the cultivators insert the grit, (usually a polished piece of mussel shell), it is always pefectly round, so as to produce a more expensive, perfectly round pearl. The brightest star in the sky (besides the Sun) is Sirius. If the grit in the centre of the pearl is a perfect sphere, then the jeweller knows it is cultivated. Although Polaris is a relatively bright star and is conspicuous since no other stars of similar brightness are close to it, it is nowhere near the brightest; it is actually the 46th brightest star in the night sky. One way that jewellers can determine whether a pearl is cultivated or natural is by x-raying the pearl. Polaris's fame as the North Star has given rise to a persistent misconception that it is the brightest star in the sky.

In general, cultivated pearls are less valuable than natural pearls, and imitation pearls are the least expensive. Polaris is not normally visible from the Southern Hemisphere. Ringed pearls are characterized by concentric ridges, or rings, around the body of the pearl. One can also follow the central point of the W shape of Cassiopeia. They are also commonly seen in necklaces. In the Northern Hemisphere, it is easy to find Polaris by following the line traced from Merak to Dubhe (β and α Ursae Majoris, also known as the Pointers), the two stars at the end of the bowl of the Big Dipper. Baroque pearls have a different appeal to them than more standard shapes because they are often highly irregular and make unique and interesting shapes. Polaris is due to become an even more accurate pole star in the near future — the distance between Polaris and the pole will reach a minimum (of just under 1/2 degree) in 2100.

Drop and pear shaped pearls are sometimes referred to as teardrop pearls and are most often seen in earrings, pendants, or as a center pearl in a necklace. Other stars have been the pole star in the past and will be again in the future, including Thuban and Vega. Button pearls are like a slightly flattened round pearl and can also make a necklace, but are more often used in single pendants or earrings where the back half of the pearl is covered, making it look like a larger, round pearl. This is due to precession of the equinox over thousands of years. Semi-rounds are also used in necklaces or in pieces where the shape of the pearl can be disguised to look like it is a perfectly round pearl. Although Shakespeare wrote "I am as constant as the northern star", Polaris will not always be the pole star. Perfectly round pearls are the rarest and most expensive, and are generally used in necklaces, or strings of pearls. Only twice during every sidereal day does Polaris accurately define the true north azimuth; the rest of the time it is only an approximation and must be corrected using tables.

Pearls come in eight basic shapes: round, semi-round, button, drop, pear, oval, baroque, and ringed. At present, Polaris is slightly under 1° away from the pole of rotation (about twice the apparent diameter of the Moon) and hence revolves around the pole in a small circle almost 2° in diameter. Irregular shaped pearls are often used in necklaces. The antiquity of the use of this star is attested to by the fact that it is found represented on the earliest known Assyrian tablets. Teardrop-shaped pearls are often used in pendants. Therefore, it makes an excellent fixed point from which to draw measurements for celestial navigation. Large, perfectly round pearls are rare and highly valued. Because it lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole—the north celestial pole—Polaris is apparently motionless from the Earth, and all the stars of the Northern sky appear to rotate around it.

All factors being equal, however, the larger the pearl the more valuable it is. Other names include the "North Star", the "Lode Star", or sometimes "Polaris Borealis". Among those attributes, luster is the most important differentiator of pearl quality according to jewelers. "Polaris" comes from Stella Polaris, the Latin form of its common name "Pole Star." The Greek name Cynosura (Κυνόσουρα) means "tail of the dog" and is the source of the English word cynosure. The value of the pearls in jewelry is determined by a combination of the luster, color, size, lack of surface flaw and symmetry that are appropriate for the type of pearl under consideration. The nearer dwarf star is in an orbit of only 20 AU (3 billion km) from Polaris A, explaining why its light is swamped by its close and much brighter companion. Cultured pearls are also produced using abalone. In January 2006, NASA released images from the Hubble telescope, directly showing all three members of the Polaris trinary system.

Freshwater pearls are characterized by the reflection of rainbow colors in the luster. In 1929, it was discovered by examining the spectrum of Polaris A that it had another very close dwarf companion (variously α UMi P, α UMi a or α UMi Ab). In the 1990s, Japanese pearl producers also invested in producing cultured pearls with freshwater mussels in the region of Shanghai, China, and in Fiji. Polaris B can be seen with even a modest telescope and was first noticed by William Herschel in 1780. Japanese pearl farmers now culture a hybrid pearl mussel—a cross between the last remaining Biwa Pearl Mussels and a closely related species from China—in other Japanese lakes. Polaris is a trinary system, consisting of a large yellow Cepheid variable (α UMi A), distantly (about 2700 AU or 380 billion km) orbited by a bright yellow dwarf (α UMi B). Since the time of peak production in 1971, when Biwa pearl farmers produced six tons of cultured pearls, pollution and overharvesting have caused the virtual extinction of this animal. Much more rarely, it is called Cynosura.

The extensive and successful use of the Biwa Pearl Mussel is reflected in the name "Biwa pearls," a phrase nearly synonymous with freshwater pearls in general. It is also very close to the north celestial pole, making it the current north pole star. This lake, the largest and most ancient in Japan, lies near the city of Kyoto. Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris) is the brightest star in the constellation Ursa Minor. In 1914 pearl farmers began culturing freshwater pearls using the pearl mussels native to Lake Biwa. Tahitian pearls (also referred to as Titian pearls) are also another South Sea pearl.

Australia is one of the most important sources of South Sea pearls. Sizes up to 14 mm in diameter are not uncommon. South Sea pearls are characterized by their large size and silvery color. One of the largest pearl-bearing oysters is the Pinctada maxima, which is roughly the size of a dinner plate.

In the past couple of decades, cultured pearls have been produced with larger oysters in the south Pacific and Indian Ocean. The original Japanese cultured pearls, known as Akoya pearls, are produced by a species of small oysters no bigger than 6 to 7 cm in size, hence Japanese pearls larger than 10 mm in diameter are extremely rare and highly priced. Oysters which survive the subsequent surgery to remove the finished pearl are often implanted with a new, larger nucleus as part of the same procedure and then returned to the water for another three years of growth. Along with a small scrap of mantle tissue from another oyster to serve as an irritant, it is surgically implanted near the oyster's genitals.

The nucleus is generally a polished bead made from mussel shell. This mariculture process was first developed by Kokichi Mikimoto in Japan, who was granted a patent for the process in 1896. The pearls are usually harvested three years after the planting, but it can take up to as long as six years before a pearl is produced. Now, however, almost all pearls used for jewelry are cultured by planting a core or nucleus into pearl oysters.

In fact, in a haul of three tonnes, only three or four oysters will produce perfect pearls. Not all natural oysters produce pearls, however. Divers manually pulled oysters from ocean floors and river bottoms and checked them individually for pearls. Before the beginning of the 20th Century, pearl hunting was the most common way of harvesting pearls.

. Black pearls, frequently refered to as Black Tahitian Pearls are highly valued because of their rarity; the culturing process for them dictates a smaller volume output due to rejection by the oysters. Pearls are usually white, sometimes with a creamy or pinkish tinge, but may be tinted with yellow, green, blue, brown, purple, or black. The iridescence that some pearls display is caused by the overlapping of successive layers, which breaks up light falling on the surface.

The unique luster of pearls depends upon the reflection and refraction of light from the translucent layers and is finer in proportion as the layers become thinner and more numerous. This combination of calcium carbonate and conchiolin is called nacre, or as most know it, mother-of-pearl. As a response to an irritating object inside its shell, the mollusk will deposit layers of calcium carbonate (CaCO₃) in the form of the minerals aragonite or calcite (both crystalline forms of calcium carbonate) held together by an organic horn-like compound called conchiolin. Pearls are formed inside the shell of certain bivalve mollusks.

Pearl is valued as a gemstone and is cultivated or harvested for jewellery. A pearl is a hard, rounded object produced by certain mollusks, primarily oysters.