Amethyst is a purple variety of quartz ( SiO < sub > 2 </ sub >) and owes its violet color to irradiation, iron impurities ( in some cases in conjunction with transition element impurities ), and the presence of trace elements, which result in complex crystal lattice substitutions.

The color of amethyst has been demonstrated to result from substitution by irradiation of trivalent iron ( Fe < sup > 3 +</ sup >) for silicon in the structure, in the presence of trace elements of large ionic radius, and, to a certain extent, the amethyst color can naturally result from displacement of transition elements even if the iron concentration is low.

Dendritic agates have fern like patterns in them formed due to the presence of manganese and iron oxides.

The presence of iron in the olivine or serpentine reduces the efficiency of sequestration, since the iron components of these minerals break down to iron oxide and silica ( reaction 4 ).

The intense heat of the flame made necessary the presence of heat shields made of iron ( βουκόλια, boukolia ), which are attested in the fleet inventories.

The iron chemistry in this previous classical model was elegant, but the required presence of the required diamagnetic high-energy singlet oxygen was never explained.

The locals also claimed that " the same spring is said to turn wood into iron ", but Defoe put this down to the presence of " copperas " in the water.

Iron's very common presence in rocky planets like Earth is due to its abundant production as a result of fusion in high-mass stars, where the production of nickel-56 ( which decays to the most common isotope of iron ) is the last nuclear fusion reaction that is exothermic.

Blacksmiths are credited with magical powers in many parts of the world, and it is significant that the Boudas are workers in iron and clay ; in the Life of N. Pearce ( i. 287 ) a European observer tells a story of a supposed transformation which took place in his presence and almost before his eyes ; but it does not appear how far hallucination rather than coincidence must be invoked to explain the experience.

The presence of iron as Fe < sup > 3 +</ sup > and Fe < sup > 2 +</ sup > helps understand oxygen fugacity, which in turn is a significant factor in petrogenesis.

In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture.

The reactivity of the iron center may be improved by the presence of the phenolate ligand of Tyr357 in the fifth iron ligand, which can assist in the oxidation of the Fe ( III ) to Fe ( IV ).

It weathers to iddingsite ( a combination of clay minerals, iron oxides and ferrihydrites ) readily in the presence of water.

The end of the 18th century saw the beginnings of the Industrial Revolution, and the presence of iron ore, limestone and large coal deposits in south-east Wales meant that this area soon saw the establishment of ironworks and coal mines, notably the Cyfarthfa Ironworks and the Dowlais Ironworks at Merthyr Tydfil.

The increasing presence of long-distance trade, an increase in local conflicts, and the introduction of bronze and iron metallurgy are trends denoting the end of the Mumun around 300 BC.

Liquid water was prevalent, and deep oceanic basins are known to have existed by the presence of banded iron formations, chert beds, chemical sediments and pillow basalts.

At that time when dietary iron may have been scarcer than today, the presence of the mutant allele may have provided an evolutionary or natural selection reproductive advantage by maintaining higher iron levels in the blood.

Each patient with the susceptible genotype accumulates iron at different rates depending on iron intake, the exact nature of the mutation and the presence of other insults to the liver such as alcohol and viral disease.

Carbon monoxide binds to iron in an end-on fashion, and is hindered by the presence of the distal histidine, which forces it into a bent conformation.

In 2009, India's Chandrayaan-1 satellite, NASA's Cassini spacecraft and the Deep Impact probe have each detected the presence of water by evidence of hydroxyl fragments on the Moon.

However, the presence of old weapon and tool fragments ( specifically, polished axes cut through shale and remnants of cooking and fishing ) in the country has been interpreted as a possible indication of a large human presence during the Upper Paleolithic period ( 15, 000 to 10, 000 BC ), or at the minimum, the Neolithic period.

The oldest evidence of human presence on Taiwan consists of three cranial fragments and a molar tooth found at Chouqu and Gangzilin, in Zuozhen District, Tainan.

The group was taken to a pre-emptied barn, killed and buried along with fragments of the monument, while most of the remaining Jews, estimated at around 250 to 300 ( IPN final findings ), including many women and children, were led to the same barn later that day, locked inside and burned alive using kerosene from the former Soviet supplies ( or German gasoline, by different accounts ) in the presence of eight German gendarmes, who shot those who tried to escape.

Fab and F ( ab ') 2 antibody fragments are used in assay systems where the presence of the Fc region may cause problems.

These fragments may also be desirable for staining cell preparations in the presence of plasma, because they are not able to bind complement, which could lyse the cells.

Part of the reason for three Gospels is the presence of differences in the surviving fragments, particularly the presence of three separate accounts of Christ's baptism, while another factor was the scepticism towards the reliability of the evidence of Jerome.

The reason why somebody had assembled this complex mixture of chemicals is unknown ; equally puzzling was the presence of fragments of glass and brush bristles, suggesting that someone had dropped a bottle of this substance and then swept it up into a new container.

This pre-computation typically involves creation of bitstrings representing presence or absence of molecular fragments.

The performance of such keys depends on the choice of the fragments used for constructing the keys and the probability of their presence in the database molecules.

The subsequent Roman presence, after the conquest carried out by the legions under command of General Publio Carisio ( the X Gemina and the V Alaudae ), left imprints such as several coins and ceramics fragments, as well as probably the origin of the names of some small villages such as Vega de Ouria, likely due to the presence of some river gold working in that time.

These fragments were similar to but distinct from known herpevirus sequences indicating the presence of a new virus.

The presence of soil layers in the basalt with included charcoal and conglomerate fragments of gneiss indicate that much of the plateau was above sea level as what is termed a microcontinent for three periods between 100 million years ago and 20 million years ago.

Silence walked at Pamela's side, its presence numbingly close, yet too far for her to hear.

This caution has been particularly noticeable in a tendency of retailers and distributors to shift the inventory burden back on the supplier, and the fact stocks at retail are low in many lines has escaped attention because of the presence of higher stocks at the manufacturing level.

Yet you feel the orchestra is near at hand, and the individual instruments have the same firm presence associated with listening from a good seat in an acoustically perfect hall.

One sample, which had been exposed to the atmosphere after evacuation at 375-degrees-C, showed the presence of adsorbed water ( about 0.3 wt ) ) as evidenced by a weak resonance line which was very narrow at room temperature and which disappeared, due to broadening, at low temperature.

Failing to heed the lesson so clearly contained in the satellite treaties, President Truman re-declared the Cold War on March 12, 1947, in the Truman Doctrine, exactly one week after the Herald Tribune editorial was written, and a year after the Cold War had been announced by Churchill at Fulton, Missouri, in Truman's presence.

No wonder that Pozzatti and I had at times difficulty in remembering the real purpose of our presence, namely, Cultural Exchange.

And at his last commencement, in that year, Dr. Tucker and Dartmouth were honored by the presence of distinguished academic visitors attesting to the new stature of the college.

This method of countin' was usually done at the request, and in the presence, of a representative of the bank that held the papers against the herd.

But for my presence, they would have been at each others' throats.

There is an extraordinary sense of presence in all of these recordings, apparently obtained at least in part by emphasizing the middle and high frequencies.

The economic purpose of the Chaco road system is shown by the presence of luxury items at Pueblo Bonito and elsewhere in the canyon.

" Lincoln died around 7: 00 A. M .; Johnson's swearing in occurred at 11: 00 that morning with Chief Justice Salmon Chase presiding in the presence of most of the cabinet.

While he was busy with the burial ceremonies for his brother, the Danes defeated the English in his absence at an unnamed spot, and then again in his presence at Wilton in May.

In 895, Arnulf summoned both Charles and Odo to his presence at Worms.

But, there is no evidence whatever of a Greek presence on the west coast and the Ionians at Aleria on the east coast had been expelled by the Etruscans long before Roman domination.

The reaction is carried out under pressure at a temperature of 200 ° C in the presence of a metallic catalyst.

Field evaporation is an effect that can occur when an atom bonded at the surface of a material is in the presence of a sufficiently high and appropriately directed electric field, where the electric field is the differential of electric potential ( voltage ) with respect to distance.

Further separation was carried out in the presence of a citric acid / ammonium buffer solution in a weakly acidic medium ( pH ≈ 3. 5 ), using ion exchange at elevated temperature.

By a further act of 1541 — which was not repealed until 1845 — artificers, labourers, apprentices, servants and the like were forbidden to play bowls at any time except Christmas, and then only in their master's house and presence.

Even the critic G. R. Driver recognized that " the presence and popularity of the Daniel manuscripts at Qumran " conflicted " with the modern view which advocates the late dating of the composition of Daniel ".

It was recorded in her presence on October 21, 1958, at Decca's Pythian Temple, with Dick Jacob, Coral-Brunswick's new head of Artists and Repertoire, serving as both producer and conductor of the 18-piece orchestra, which included members of the New York Symphony Orchestra, NBC Television's house orchestra and Abraham " Boomie " Richman, formerly of Benny Goodman's band.