Topaz Enlarge
June 1 1810. publishd by Jas Sowerby London.
British Mineralogy
CCCLXIII
Argilla Topazius

Topaz

  • Class 2. Earths.
  • Order 1. Homogeneous.
  • Gen. 1. Argilla.
  • Spec. 11. Topaz.
  • Syn.
    • Occidental Topaz. Kirw. 1. 254.
    • Topaz. Emmerl. 1. 73.
    • Topaze. Haüy, 2. 504.
    • Borax, gemma nobilis Topazius. Linn.

Topazes have been found in the island of Ceylon, the Brazils, Bohemia, Saxony, Cornwall, &c. In Lesser Asia a rose-red variety was discovered by our good friend Mr. Hawkins before mentioned in this work; and since that they have been found in New Holland, and in Scotland. The specimen figured is from Cairn-Gorum, and was lent me by its kind possessor Thomas Allan, esq. It is the more curious, as part of it is of a fine light blue colour*, and part cinnamon-coloured, with a beautiful soft glowing warmth; and the disposition of the two colours seems almost to explain the nature of the crystallization. I thought therefore it would be instructive to give a figure of the base of the crystal, showing nearly the form the colours are disposed in. See the left hand lower figure. The right hand geometrical figure shows the faces of the crystal, the top and bottom being primitive, with which the fracture agrees. This figure is about the size of a crystal, from the same place, in the possession of R. C. Fergusson, esq. which has the same colours as the other, though more dilute. This is perhaps peculiar to the Scotch Topazes, as may he verified hereafter; for they may prove to be more common than is now expected. The people that gather them, often call the Cairn-Gorum Quartz by the name of Topaz (being a stamp of value); and they are sold indiscriminately. This crystal, which bears the marks of many a violent storm on its worn edges, has been broken through the primitive base transversely to the column, and is fractured in the laminæ, showing the order of the prismatic colours very well; and by pressure these colours are spread wider, which shows that there is a great degree of elasticity in the laminæ.

The geometrical figure is to show more distinctly the faces that are in the principal figure, which is rather remarkable for not having the faces so constantly opposite as they generally are in Topazes. Between the large pentangular face (n of Haüy) and the primitive apex, there is a parallel face, not mentioned by Haüy, and perhaps one opposite, but so small that, as the crystal is rounded by attrition, we are not sure it is there. At the lower corner is the triangular face (c of Haüy), on the nearest side, but I think not on the opposite; but it is marked in the outline to show where it would come. The two faces on each acute side of the prism are primitive, on the top of which a want of increase forms two faces (o, o, of Haüy).

The larger four sides of the prism are the faces l of Haüy. The figure within the outline shows the position of the primitive faces and fracture. The obtuse angle of the base is 124° 22′ according to Haüy.

The value of Topazes depends much on their colour and lustre. It is supposed to be the hardest substanec known, excepting Diamond and Corundum. Their hardness, fracture, and brilliancy may have confounded them with Diamonds; and it is said that Saxon Topazes are heated to give them a white transparency, and are then actually sold for Diamonds—it is even suspected by some that the reputed Portuguese Diamond may be only a Topaz. Spec. Grav. 3.464–3.564.

The Saxon Topaz analysed by Vauquelin afforded him'

Alumina 49
Silica 29
Fluoric Acid 20
Loss 2
100

The Brazilian gave

Alumina 47
Silica 28
Fluoric Acid 17
Oxide of Iron 4
Loss 4
100
  • * Topazes are of different colours, from deep wine yellow through reddish yellows to greenish and rarely blueish—also colourless and transparent.
  • † I believe the same done with some Diamonds. It is very usual with the dark Cairn-Gorum Crystals. See Descr. tab. 102.
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