N. America claims to afford diamonds in California and Oregon, where they are occasionally found in the gold-sluices; their size is very small. They are also mentioned in Arizona, Georgia, N. Carolina, and in the Sierra Madre, near Acapulco (Mexico).
Brazil was the chief source of diamonds at the close of last and opening of this century. The most productive districts are Diamantina (Tejuco), in Mines Geraes, Diamantino, in Matto Grosso ; also on the Rio Claro, on the Rio Tibagy, in Sao Pedro do Rio Grande do Sul, and in Sao Paulo. The total area of distribution is yet far from being known. The stones are sought for in the cascalho, a loose deposit of grave], clay, and quartz lumps, containing gold, and supposed to be the detritus of " itacolumite," a quartzoze mica-slate, or metamorphosed sandstone, in which diamonds also occur. This latter is too hard to repay working, and operations are confined to washing the cascalho, as if for gold. The stones are mostly small, averaging but little more than 1 carat. The exports from Bahia in 1878-9 were 8269 grrn., value 709,324,000 reis (1000 reis = 4s. 50.), of which, France took 574,854,400 ; Great Britain, 132,587,200; and Portugal, 1,882,300.
Several of the Australian Colonies yield diamonds, though not of great size nor in large number. The most noted localities are the Macquarie and Mudgee river-valleys, and Bingera, all in New South Wales. The finds are in the auriferous drift of dead rivers, overlaid by Pliocene basalt. They also occur in the Cudgegong River, 19 miles north-west of Mudgee. In South Australia, they are found near Echunga, about 20 miles south-east of Adelaide ; in Victoria, at Beechworth and Collingwood Flat. They are reported from N.-E. Gippsland and from New Zealand.
Emerald (FR., Emeraude ; Gem., Smaragol).—Composition, 65 per cent. silica, 14 alumina, 13 glucina, 3.5 oxide of chromium, 2.5 lime ; hardness, 7.5; sp. gr., 2.7 ; colour, rich deep green ; somewhat brittle, transparent to subtranslucent. Europe is said to possess emeralds in Norway and Austria. In Asia, they have been found in the Urals and Altai Mountains, in Burmah, and on the Siberian frontier of China. African emeralds are found in mica-slate beds in the Sahara, and at the junction of the Harrach and Qued Bouman Rivers, in Algeria. The principal modern source of the gem is in S. America, between the mountains of New Granada (Colombia) and Popagan. The most celebrated mine is that of Muzo, in the Tunka Valley, about four days N.-N.-W. of Bogota. The formation is a secondary limestone, containing veins of
calcareous bitumen, in which the gems are found imbedded. Mining operations are necessary, and the broken-down material is washed in ground sluices. The production is very variable, and no statistics exist. Stray emeralds are reported from Victoria and New South Wales.
Lapis Lazuli (FR., Pierre ollaire ; GER., Topfstein, Lawetzstein).—Composition, 45-50 per cent. silica, 30-32 alumina, 9 soda, 6 sulphuric acid, with minor quantities of lime, iron, chlorine, and sulphur ; hardness, 5.5; sp. gr., 2.4 ; colour, ultramarine or fine azure-blue of varying intensity, depending, it would seem, upon the proportion of iron and sulphur. The stone occurs in Asia and S. America. A celebrated mine is in the valley of the Kokcha, in Badakhshan ; here it is met with in an unstratified limestone, and is extracted by heating the surface of the rock so that it can be flaked off by smart blows till the stone is exposed. Another source is the shores of the Shudauk, near the Baikal Lake ; also in many parts of China, and reputedly on the Indus. In the Cordillera of the Andes, near the sources of the Cazadero and Vies, tributaries of the Rio Grande, the gala is found in a thick stratum of limestone, accompanied by small quantities of iron pyrites.
Opal (FR., Opals ; GER., Opal).—Composition, 90-95 per cent. silica, 5-10 water, with traces of iron, potash, soda, lime, alumina, &c. ; of various colours and many varieties ; the noble or precious opal, the only one to be considered here, exhibits a beautiful play of colour by refracted and reflected light. The only two sources of precious opal are Hungary and Mexico, the product of the former being by far the more valuable. The Hungarian mines are situated at Dubrick and Caorvenicza in tho Carpathians. The stones occur as drums, irregularly scattered throughout the interstices of an andesite, or trachytic lava, forming the mass of the mountain. These opals vary in value from 11. to 51. a carat, and even higher, and are almost the only ones employed by jewellers. The Mexican and Honduras stones come from Esperanza, Amealeo, and Real del Month, occurring in a porphyritic formation. They are beautiful when new, but soon lose their beauty, and are worth only a few ponce a carat. S. Australia is said to afford a few specimens resembling the Hungarian ; and some of particular beauty are reported from Beechworth, Victoria.