-161 Earths. M I N E R A L O G Y, Earths. 
generally femitranfparent, rarely diaphanous, The fur- 
face is rough j fracture perfectly even, though Ibinetimes 
palling into the fine fplintery or imperfectly conchoidal. 
Specific gravity, from 2-586 to 2-655. Contains filex 84, 
alumine mixed with iron 16. Bergman. 
Almoft all the appearances of calcedony juftify the idea 
that it has been formed by ftalagmitic depolltion ; parti¬ 
cularly the bliftered or mamillated appearance fo com¬ 
monly oblervable on its furface. A very lingular ltalac- 
titic form of calcedony occurred fome years lince in one 
of the Cornifti mines : it is nearly opake and colourlefs ; 
and, from its clofe relemblance to an agglutinated mals 
of the cylindrical bones of fmall birds, it has been deno¬ 
minated Jkeletou-calccdoiuj. But, although calcedony ap¬ 
pears to have been feparated from water in as great a Hate 
of purity as any mineral, and often to have been placed 
under circumftances as favourable as pofiible to cryftalli- 
zation, it never fhows the lead tendency to an internal 
cryftaliine arrangement of its particles: its frafture is al¬ 
ways perfectly compact. Cryftals, indeed, the fubftance of 
which is calcedony, are often met with ; but they are al¬ 
ways pleudomorphous; and very frequently they are not 
folid, which is a Itrong argument in favour of their ftalag¬ 
mitic origin. Their furface is rarely finooth. The deri¬ 
vation of calcedony from the common black flint may be 
traced by very delicate gradations, and fometimes in the 
fame-fpecimen ; for, upon breaking a nodule of the flint, 
a cavity is not unfrequently dilcovered, the furface of 
which conlifts of irregularly-bliftered calcedony; the fub- 
ftance of the flint infenfibly palling into that of calcedony. 
Calcedony is frequently contained in rocks of the bafaltic 
genus, in nodular mafles. Thefe mafles, which are in ge¬ 
neral of a light bluilh-grey colour, are often hollow, and 
fet with amethyltine quartz cryftals. 
3. Chalcedonius caeruleus, or blue chalcedony : blue; 
of a flatter texture, femipellucid, of a common form, not 
falling fpontaneoufly into fragments. Found in the Feroe 
Iflandsr, the fhores of Scotland, in Saxony, Bohemia, Hun¬ 
gary, and Tranfylvania. Colour fometimes verging to 
cinereous or milky, fometimes clear Iky-blue, or between 
violet and lavender-blue, or fapphirine : in other refpedts 
it refembles the preceding. 
4. Chalcedonius niger, or black chalcedony: black, 
dull red when oppofed to a ftrong light, of a flatter tex¬ 
ture, fomewhat diaphanous. Found near Chemnitz in 
Saxony, imbedded in porphyry, 
5. Chalcedonius frii'cus, or brown chalcedony : brown ; 
of a texture fometimes flatter, fometimes conchoidal, dia¬ 
phanous. Found detached at the river Tom in Siberia, 
and near Chemnitz in Saxony, imbedded in porphyry. 
6. Chalcedonius luteus, or yellow chalcedony : pale 
yellow ; of a flat -texture, verging to the conchoidal. 
Found detached near the river Tom in Siberia, and in 
Ceylon, Hungary, and Saxony. Colour fometimes wine- 
yelloW, fometimes wax or honey yellow. 
7. Chalcedonius carneolus, red chalcedony, or carne- 
lian : blood-red, femitranfparent, of a perfe 611 y-conchoi- 
dal texture: Found in Arabia and Hinuooftan, Egypt, 
and various parts of Europe, generally in roundilh pieces, 
arid alto in layers in agate. Colour various flradesof red ; 
rarely opake, and fometimes turbid with a few cloudy 
iiiades; outer furiace rough and uneven, the fragments 
indeterminately angular and fharp-edged. Specific gra¬ 
vity, from 2-6 3 to 27. 
The word cornelian (fays Dr. Kidd) is perhaps expref- 
five of the flefh-colour of this mineral. Carneiian, often 
called 'cornelian., is fometimes imported from India in the 
form of irregularly-shaped nodules, very much resem¬ 
bling Scotch pebbles ; afid a tranfverfe lel'tion of thefe 
nodules very"often prelim ts.a iiaular arrangement of con¬ 
centric lines with that of the- common agate ; of which it 
is now generally coniidercd a variety. The nodules of 
canvCian l a ve generally a reddiih-brown coloured cruft. 
The rcoli ili-iicing colours of the. carneiian are, that deep 
fiehwed peculiar to it;, and a hluiflt white, very like the 
colour produced by the mixture of a little milk with « 
great deal of water. Arabia and India produce the moft 
beautiful carnelians: but the greateft number come from 
Germany. 
8. Chalcedonius fardus, white carneiian, or fard: whit- 
ifli, fometimes variegated with blood-red dots or ftreaks. 
It is found in India, the Palatinate, and Sardinia ; from 
which laft place fome derive the name ; others from its re- 
femblance in colour to the flefh of the anchovy (jardina) 
when falted. Dr. Kidd mentions a gem of this kind which 
was cut as a cameo ; the device was a head of Bacchus; 
and it was- fo contrived, that the red part reprefented 
the cheeks, the reft being entirely white. 
9. Chalcedonius dendriticus, or Mocha-ftone. Ac¬ 
cording to fome, it is called Mocha-ftone from the place 
of its importation ; for they fay that it is only hupped at 
Mocha, after having been brought there from a diftance ; 
but it occurs in Iceland, the Palatinate, and other parts of 
Europe. According to others, the term is derived from 
a word Signifying mofs; this variety of the agate fome¬ 
times appearing to contain that vegetable. The ground 
of the Mocha-ftone is generally femitranfparent, and of 
a light brownifli honey colour ; with reprefentations of 
the form of leaves, mofs, and vegetable fibres of a brown 
and black colour. 
10. Chalcedonius maculatus, or plafma: marked with 
fpots differing in colour and in degree of tranfparency. 
Found in Ceylon, the Ferro Blands, Italy, and the Pala¬ 
tinate, according to Linnasus ; but Dr. Kidd fays, that 
its native fituation is unknown, having been hitherto 
found only among the ancient ruins of Rome. 
11. Chalcedonius fafeiatus, the fardonyx. This term 
was originally applied to a ftone, the colour of which was 
compounded of the onyx and fard. Carnelians are fome¬ 
times of that yellowifh-red colour which would reiult 
from an intimate mixture of the two ; but the fardonyx 
of modern jewellery is a variety of the onyx of the pre- 
fentday. The true fardonyx, however, is found in Cey¬ 
lon, Ferro and Iceland, Bohemia and Saxony. Colour 
grey, fomewhat pellucid, with milk-white diaphanous 
bands, rarely with thofe that are rofy or green, fometimes 
bluifh or blood-red, with white or grey bands. 
12. Chalcedonius onyx, the onyx : breaking into con¬ 
centrically-cruftofe fragments differing in colour and de¬ 
gree of tranfparency. Found in the Eaft Indies, Siberia, 
Bohemia, Portugal, and Saxony, in thicker or thinner 
fragments, and fometimes in pebbles. Colours grey and 
black, white flefh-colour and black, red and white, white 
and grey, various ftiades of yellow, alternating in ftthdry 
manners, generally in concentric circles: it lofes its co-’ 
lour in the fire, and cracks and breaks if the heat be Bid¬ 
den or violent: it is the hardeft of all its genus. Specific 
gravity, from 27 to 2-6. 
The value of this l’ubftance in jewellery depends parti¬ 
cularly upon the difference in the colour of the ltrata; arid, 
in engraving heads or whole figures upon the onyx, in 
relievo, as it is technically called', the upper ft in turn is 
made ufe of for the reprelentation of the intended figure; 
the lower, for the bale on which it is to reft. In engrav¬ 
ing in intaglio, this order is reverfed, and the figure is cut 
in the lower ftratum. In the former in fence- the figure is 
convex, and projects from the inferior ftratum; in the 
latter it is concave, and finks in from the luperior ftratum. 
In fome varieties the ltrata are four or five, or even more, 
in number; and the colours, of very different lhades; 
thefe, when cut into a fpherical lhape, clofely referable 
the eyes of various animals; the upper ftratum being- 
brought to a point aniweringto the pupil. From the re- 
fembtance of lpecimens of this kind to the eyes of different 
animals have arifen various terms expreflive of the parti¬ 
cular refemblance; as leucophtkidmus, erytlirophthalmus ; 
and, with a reference to the number of the zones, dioph- 
thalmus and triophthalmus; again, from a refemhlance- 
to the eyes of particular animals, lycophthaliuus, ai- 
gophthalmus, &c. They are fometimes called by the 
Italians 
