FACETIP – TOPAZ
January 19, 2019By Duncan Miller
Topaz is a rather under-rated gemstone. This perhaps it due to the fact that pure, colourless topaz is relatively plentiful. Much of it is irradiated and then heat-treated to produce various intensities of bright blue. Natural blue topaz tends to be much paler, although dark blue stones do occur naturally. These are rare and hence more valuable. Natural topaz occurs in a wide variety of colours, including light green, yellow, orange and pink. The famous orangey-pink topaz from near Ouro Preto in Brazil, known in the gem trade as ‘Imperial topaz’, usually is heavily included and clean stones command a high price. Unhappily, some strongly coloured topaz fades in sunlight.
Large, clear crystals of so-called ‘Silver topaz’ occur in the pegmatites of the Klein Spitzkopje and the miarolitic cavities in the Erongo granite. This inexpensive material is readily available from artisinal miners and local dealers. Clean rough can produce very brilliant stones because of the high lustre.
Faceting topaz requires attention to the crystallography. Topaz crystals often are elongated in the direction of the c-axis, and the prism faces (the ‘side’ faces) often have striations parallel to the c-axis. This is one of the characteristics that distinguish topaz crystals from quartz, in which the prism faces are striated perpendicular to the c-axis. Other distinguishing features are the rectangular or rhombic horizontal cross-sections of well-formed topaz crystals, as opposed to the hexagonal cross-section of quartz. Topaz is both denser and harder than quartz, so a specific gravity test or a Mohs hardness test can distinguish more irregular rough. Topaz has one direction of easy cleavage perpendicular to the c-axis of the crystal. Careful inspection of water-worn chunks often reveals cleavage traces either inside the crystal or as tiny, regular step fractures in chipped areas.
Locating the cleavage direction is important for orientating the stone to be cut. You want to avoid any facet on or near the cleavage. The rule of thumb is to orientate the table of the stone at least 5˚ off the cleavage, and 7˚ to 10˚ is better. An alternative with elongated rough is to orientate the cleavage 10˚ off the vertical in your stone, to avoid having the cleavage near any girdle facet.
Preforming can be done on a sharp coarse lap with light pressure, to avoid creating cleavage cracks. Because topaz is relatively hard (8 on the Mohs scale) a really good pre-polish is necessary, at least a 3000 mesh finish. I prefer 8000 mesh diamond on copper. Polishing with aluminium oxide (Linde A) is possible, but slow. Polishing with 60 000 mesh or 100 000 mesh diamond on a metal lap like Batt or tin/lead, or a Diamatrix composite lap, is preferable. If the cleavage gives trouble when polishing any facet, changing the direction of polishing often solves the problem.
Natural topaz from the Klein Spitzkopje in Namibia usually is colourless but sometimes light blue. One stone has a black tourmaline inclusion.
Orange to pink topaz crystals from Ouro Preto in Brazil are sought-after collector specimens and clean stones are valuable rarities.
A Namibian Silver topaz crystal standing on its basal cleavage face, shows its orthorhombic symmetry and vertical striations on the prism faces that distinguish it visually from quartz.
A well-polished Silver topaz makes a brilliant, durable gemstone – see examples below. (The blue tones are reflections of the sky.)
10,05 ct 9,15 ct
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