A giant 225-pound Brazilian topaz crystal, one of the largest and finest in the world, has been obtained by the Mineralogical Museum, and will be placed on permanent public exhibit about February 6, museum officers announced today.

An ordinary topaz crystal weighs but a few ounces and is about one inch or less in diameter. The Harvard topaz is eighteen inches in diameter and described as a "short prismatic" crystal. It is white in color, with inclusions of dark manganese minerals distributed in parallel streaks.

100,000,000 Years Old

Harvard mineralogists said that the crystal is probably about 100,000,000 years old, and that its formation must have required extraordinarily stable conditions for a period of a million years or more.

Topaz crystals are an aluminum silicate, formed originally by slow accretion from a hot solution. The crystals are formed ordinarily in "pockets" or hollow spaces in rock masses. Formation of good-sized crystals requires a large pocket in the rocks, and stable external conditions; temperature, for instance, must be constant within a few degrees.

Museum officials stated that although yellow topaz is most popular for gems, actually other colors are more common. The mineral is found in white, gray, green, blue, and red, as well as straw-yellow and wine-yellow.