Corsi Collection of Decorative Stones

Class XIII
Feldspars (Felspati)

Feldspar is a hard stone that appears under various colours. It has a lamella texture, and because of this formation its scales are always shiny, and also chatoyant, even when unpolished. It is found in masses, and combined with other substances. In this class I place those stones that are either entirely composed of feldspar, or that for the most part are formed from it.

Feldspars can show a number of beautiful optical effects, the result of light playing on layers of slightly different chemical composition. The milky white sheen shown by moonstone is referred to as 'schiller', and the rainbow play of colours in labradorite is termed 'iridescence' Chatoyancy, the 'cat's eye' effect, is caused by light playing across parallel fibres that are trapped inside or make up a mineral. It is not seen in any of Corsi's feldspars, but it is shown particularly well by another decorative rock not known to Corsi, a variety of quartz called tiger's eye.

Species I
Amazon stone (Pietra delle Amazzoni, Smaragdum calchedonium)

704. (169) Pliny 100 mentions twelve species of green stones, which he vaguely calls[p170] Emeralds. The finest are those from Scythia, Bactria, and Thebes; others less transparent, but still gemstones, are taken from various parts of Ethiopia, and Persia. He also places with the Emeralds the stone from Sparta, which is found on Mount Taygetus, commonly called serpentino and the only green porphyry with large crystals. Therefore it is not really amazing that the King of Babylon might have possessed an emerald three cubits wide, and five cubits high, nor that in the Egyptian labyrinth there might have been a colossal statue of emerald, and that in the temple of Jupiter there might be seen an obelisk forty cubits high, made up of only four emeralds. Pliny finally mentions another emerald called chalcedony because it used to be found near Chalcedon (Calcidonia), today Scutari, a Thracian city. Everything that the historian says about it persuades us to believe that this so-called emerald might have been the green feldspar, called Amazon stone because it is found near that river in South America. In the first place he says that it always used to be found in the vicinity of the 'copper mines', and in fact [p171] Amazon stone is coloured by oxide of copper; that it was brittle, and of indeterminate colour, similar to the neck feathers of pigeons; that it was 'scaly', much the same as the lamella texture which mineralogists recognise in Amazon stone; and that finally it appeared more or less shiny according to how that the stone was 'inclined'. We have already observed this characteristic of the varying of the extent of resplendence with inclination that is typical of feldspars. Comparing the stone with the description of it by Pliny the hypothesis seems proven, but the culmination of the proof is supplied by the chance, but informed, discovery made by the eminent Mineralogist, Earl Compton who found, in an excavation near Tivoli various pieces of the so-called Amazon stone. Some of them were engraved with various Egyptian hieroglyphics, and appeared to be fragments of a small obelisk, thus showing that such stone was not only found in Egypt, but that the ancients were familiar with it. The stone recently discovered is wholly similar to the American one by virtue of the shade of [p172] green that colours it, by belonging to the feldspars, and by often being combined with quartz amethyst, and with mica. The specimen in this collection is from the river Amazon, and for its size is (very rare).

It was a long-standing myth that amazonite (amazon stone) came from the River Amazon. It is a variety of microcline feldspar not coloured by copper as Corsi had presumed, but by trace lead, and it forms in coarse-grained rocks of granitic composition called pegmatites. Corsi points out one of the particularly distinctive features of this mineral, the small flecks of white albite feldspar which give it that scaly appearance.

Corsi's specimen would have come from the Ilmen mountains of Russia, where a large deposit had been discovered in the 18th century. In 1876, some thirty years after Corsi's death, another deposit was discovered in Colorado, USA, that was to yield exceptionally fine crystals. Since then, amazonite has been found in a number of countries.

It has been well-documented that amazonite was used as a decorative stone by the ancient Egyptians, but geologists might share the suspicion that the piece that Earl Compton had discovered, which bore Egyptian hieroglyphics, might have been a sample of 'graphic granite', an entirely natural effect in which grey quartz crystals enclosed in the green feldspar have the general appearance of Egyptian characters.

Species II
Labradorite (Pietra di Labrador)

705. (172) This fossil is generally a dark grey, which makes it nondescript at a first glance, but as soon as its position is changed in relation to the light it appears to be of extraordinary beauty because it reflects many colours that seem to emanate from its interior. The most definite colours, and those occurring most often, are dark dark-blue, green, yellow, and red verging on pink, and these change with each movement of the stone. It was first brought from the island of St. Paul near the coast of Labrador in America, and afterwards it was discovered in other places. It is found in round pebbles. Brongniart 101 called it feldspato opalino. (Very rare).

Corsi describes the characteristic iridescence of labradorite feldspar very well, and the finest specimens of this mineral are strikingly beautiful. Unfortunately Corsi's own specimen shows barely any iridescence.


Species III
Adularia (Adularia)

706. (173) Its colour is white verging on verdant green and light yellowish. Exposed to the light in different positions it shows a beautiful variability like mother-of-pearl that is sometimes chatoyant, reflecting white, red, dark blue, and green. It is also known under the name of moonstone (pietra lunare). The formation is of large laminae apparently detached from one another, and for this reason it is easily flaked, but otherwise it is hard. It is called Adularia from the ancient name of Adula that came to mean the Alpine chain where the mountain of S. Gottardo is, which is where Professor Pini discovered it. It is also known under the name of moonstone. (Very rare).

Corsi's specimen is a fine sample of adularia, a variety of orthoclase feldspar. However it does not show the schiller of a moonstone.


Species IV
Nephrite (Pietra nefrite, Marmor aequipondus)

This stone is very hard and always compact; to look at and to touch it is greasy, thus it takes a mediocre polish. Its name has been changed several times, according to various uses to which it has been put. The Romans used to call it marmor aequipondus because they valued it for making weights by means of which to regulate balances or scales. They used to make such an object round, and then two levels were made on opposite sides of the stone, in such a way that its weight could not be made smaller without the fraud being detected. All sizes of weights of this stone have been found in the excavations of Rome, beginning with the very smallest ones of an ounce, up to large ones of fifty pounds. The Aediles impressed the number corresponding to the weight on them, and they were vigilant in making sure that it was not altered. On account of their form and their weight they were used for torturing the [p175] Christian martyrs, and were called lapides martyrum. In many churches here are large nefriti set in the walls, as can be seen in S. Sabina, S. Clemente, S. Prassede, S. Pudenziana, and S. Paoloalle Tre Fontane.

Corsi i (1828) quoting from Luca Peto makes this passage clearer by saying that the ancients used to make weights from a certain black stone marked with some veins of a very deep green, and of the hardness of flint (selce). To the large ones, that went up to the weight of a hundred pounds, two rings were attached with lead, or else two hooks of iron, to the small ones only one ring or hook. There were very small units of them to the weight of five, four, and even of a single ounce.'

Writing of the Christians Corsi uses the Italian word martizzare (from the late Latin martyrizare) saying literally 'they [the weights] were used to martyrize the Christians'.

The name nephrite, or nephrite jade, has long been used for fine-grained compact actinolite or tremolite, a particularly tough mineral that makes an attractive pale to dark green semi-precious gemstone ii . 'Jade' is also the name of rarer gem mineral, jadeite. Corsi's specimens may superficially resemble jade of one kind or the other, but they are nearly all serpentines.

i. Corsi (1828) 111
ii. Price (2007) 256-257

707. (175.1) Nefrite orientale. This stone, of which the quarry is not known, is of a light apple green, without lustre, slightly transparent and of an extreme hardness. It is better known by the name of jade (giada). (Very rare).

708. (175.2) Nefrite antica. Its usual colour is deep green verging on purplish, with waves of a slightly lighter green. (Rare).

709. (175.3) Nefrite delle Alpi. Green ground, verging on black, flowered with faded yellow green. (Rare).

710. (175.4) Nefrite delle Alpi. Deep green ground with waves of an emerald green. (Rare).