Topaz
3 of the 6 encyclopedias on this shelf carry an entry for Topaz. All 3 are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
a precious stone. It is composed chiefly of aluminum, silicon, and fluorin. It has a glossy luster, but is almost as hard and as difficult to melt as a diamond. We cannot speak of a topaz color, for the gem may be colorless, or it may be of a straw-yellow, gray, greenish, bluish, or even reddish color. It is difficult to distinguish a colorless topaz from a diamond. Colored topazes are likely to fade on prolonged exposure to a strong light. The topaz occurs usually in the form of a crystal in such rocks as gneiss. Valuable gems are found in Maine, Connecticut, Colorado, Utah, and California, but chiefly in Mexico, Brazil, Japan, and Siberia. A topaz weighing 150 pounds in the rough was found near San Diego, California. It sold for $150. Cut and polished it produced a gem six inches in diameter, valued by experts at $10,000. See Gem
Collier's New Encyclopedia (1921)
a mineral, ranked by mineralogists among gems, characterized by having the luster vitreous, transparent to translucent; the color yellow, white, green, blue; fracture subconchoidal, uneven; sp. gr. 3,499. It is harder than quartz. It is a silicate of aluminum, in which the oxygen is partly replaced by flourine. It occurs massive and in crystals. The primary form of its crystal is a right rhombic prism. Topazes occur generally in igneous and metamorphic rocks, and in many parts of the world, as Cornwall, Scotland, Saxony, Siberia, Brazil, etc. The finest varieties are obtained from Brazil and the Ural Mountains. Those from Brazil have deep yellow tints; those from Siberia have a bluish tinge; the Saxon topazes are of a pale wine-yellow, and those found in the Scotch Highlands are of a sky-blue color. The purest from Brazil, when cut in facets, closely resemble the diamond in luster and brilliance.
The Encyclopedia of Founding (1892)
This precious stone is found in Saxony, Bohemia, Siberia, and Brazil, mixed with other minerals in granite rocks. its colors are yellow, bluish, greenish, lilac, and white. Electric by heat, but not by rubbing. It is composed of alumina 47.5, silex 44.5, fluoric acid 7, oxide of iron 0.5. This is for the yellow variety; their compositions vary. It occurs crystallized and in water-worn pebbles, being harder than quartz, but hardly as hard as the ruby. The yellow variety, when without flaws, is employed for jewelry. See SAPPHIRE; PRECIOUS STONES.
What Has Changed in the Last Century
The three accounts have the chemistry roughly right — Aiton's aluminium, silicon and fluorine is now written as the formula Al2SiO4(F,OH)2 — but Aiton's 1910 claim that topaz is "almost as hard and as difficult to melt as a diamond" would not pass today: on the Mohs scale topaz is the standard for 8, well below corundum at 9 and diamond at 10, and it is the mineral by which that step is defined. The biggest change is commercial rather than scientific. Nearly all the blue topaz sold today, a colour the 1892 and 1921 entries treat as a rare natural tint from Scotland or Siberia, is colourless Brazilian or Nigerian stone that has been irradiated in a reactor or accelerator and then heated, a treatment industrialised from the late 1970s that turned blue topaz into one of the cheapest gems on the market. Brazil, named by all three, is still the leading source, and the Ouro Preto district supplies the orange-pink "imperial" topaz; jewellers also had to be pushed to stop selling heat-treated yellow quartz as "topaz", a substitution now barred by trade rules.
Written for this edition, 2026.