PRECIOUS OPAL: OCCURRENCE IN MEXICO 381 



varieties is a pearl-grey opal with a play of red light ; this has no market value, but its 

 occurrence is taken as an indication of the existence of better stones. 



lb is evident that there are occurrences of opal in Honduras which are unknown to 

 ■white men from the fact that fine stones are constantly brought into the tofrns for sale by 

 Indians. These localities may extend over the border of Honduras into Guatemala, since 

 precious opal from this State is to be seen in various collections. Neither the exact localities 

 nor details connected with the mining of opal in this country are known. It is said that a 

 ■belt of opal-bearing trachytic rock extends from Honduras northward through Guatemala, 

 perhaps as far as Mexico, where opal is of frequent occurrence and is mined. 



The occurrences in Mexico are of some importance, especially in the mines of 



Esperanza, where precious opal is so common that specks of it are often to be seen in the 



stones of buildings. These mines are situated ten leagues north-east of San Juan del Rio 



in the State of Queretaro and extend over an area measuring thirty leagues in length and 



twenty leagues in breadth. The occurrence was accidentally discovered in 1835 by an 



agricultural labourer, but systematic operations have been in progress only since 1870. The 



precious stone is found here as elsewhere in a trachytic rock of reddish-grey colour and 



porphyritic structure, of which Ceja de Leon and Peineta among other hills are built. In 



these hills are sunk many mines remarkable for the amount and variety of the material they 



yield. Thus a single block of rock from the Simpatica mine usually contains precious opal, 



harlequin-opal, lechosos-opal (a variety presently to be mentioned), milk-opal, and fire-opal. 



One of the largest mines is the Jurado ; it is an excavation in the trachyte 150 feet deep, 



100 feet wide, and some hundred feet long. Many other smaller workings are to be seen, 



though at present but few are in active operation. 



The opal-bearing rock is sent to Queretaro, twenty-five leagues off, where almost the 

 whole of the product of the mines is cut, but in so rough a fashion that the precious material 

 is shown to very little advantage. The work is done by twenty native lapidaries in three 

 cutting works. Very little uncut material is exported. In the mines themselves about a 

 hundred Indians are employed; 50,000 cut stones are sold yearly, and with improved 

 methods the production could be easily doubled. Large numbers are sent to the United 

 States, where, in some districts, they are sold to travellers at the railway stations as home 

 products. Others are exported to Europe, especially to Germany, where they are used in 

 cheap jewellery. Central American and Mexican opals are very variable in price, but are 

 always worth far less than Hungarian stones. The cheapest stones are worth no more than 

 a few cents apiece, while the best will fetch a hundred dollars ; parcels containing a hundred 

 or more pieces of opal are often sold at less than ten cents apiece. Higher prices are 

 paid for exceptionally fine stones, but they do not equal the sums paid for Hungarian opals. 

 Mexican precious opal often occurs in common opal in layers which are too thin for 

 cutting. Cavities in the mother-rock are filled completely, or sometimes to only one-half or 

 two-thirds of their extent, with these masses of opal. The different varieties are disposed 

 in horizontal layers, and the uppermost is often a botryoidal layer of glassy hyalite. Such 

 a mode of occurrence clearly indicates that the opal has been deposited from water containing 

 silica in solution. 



Precious opal occurs here in a considerable number of varieties : all are remarkable for 

 the intensity of the colours reflected from their surfaces, and in this respect are comparable 

 to Hungarian stones. Mexican opals frequently show extensive patches of one. uniform 

 colour, while over the whole surface of some stones there is a play of only a single colour, 

 red, green, or yellow, which either remains unchanged as the stone is moved about or passes 

 into other colour*. 



