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Ch. 13: Olivine - Sphene

Ch. 13: Olivine - Sphene Page of 311 Ch. 13: Olivine - Sphene Text size:minus plus Restore normal size   Mail page  Print this page
PRECIOUS STONES.
263
of the spectrum are: for a crystal from Siberia, 1.619 and 1-610, while for the F line in the blue portion of the spectrum they are 1.628 and 1-619. Thus there is but little "fire" in a cut specimen of Topaz, but coloured varieties show distinct dichroism. Before the blowpipe it is infusible, but the effects of heat are nevertheless impor­tant in the case of Topaz. In 1755 a French jeweller named Dumelle found that on gradually heating a yellow
Topaz the colour changed to a pink, and this effect has been made use of in the artificial production of a pink colour, a process now known as "pinking" or "burning," and one by which most of the pink stones now sold are produced. To effect this change the mineral must be heated and cooled again very gradually to avoid producing flaws; it may be heated in a sand bath, or packed with charcoal in a small crucible, or embedded in clay and then baked, or, finally, it may be wrapped in several layers of
Ch. 13: Olivine - Sphene Page of 311 Ch. 13: Olivine - Sphene
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