Diamond macle
In a recent post (see http://on.fb.me/19XRzNF) we discussed the idea of twinned crystals, illustrated with three beautiful pyrite crystals. Macles (from an old heraldic word for lozenge) are a more complex form of twinning, in this case representing two octahedral crystals that have grown together, ending up with this triangular slab shape.
They are very hard to cut, since the twin plane that separated the crystals within the gem marks a change in orientation, and hence a change in hardness known to diamantaires as a knot. Diamonds can only be cut because they have different hardness in different direction, so the hard direction of one diamond is used to cut the softer one of another. Working with macles is a subtle and risky business, since accumulated strain from past tectonic pressures within the ston can cause it to explode when the knot is reached.
Jewellers have started using these diamonds in the rough as the centre tone of unique design pieces, and I much prefer this to a faceted brilliant.
The triangles on the surface of the crystal are known as trigons, and very common in diamond. They reveal the crystal structure, being formed as the crystal gets partially dissolved away during its rise to the surface in its carrier kimberlitic magma. Cheats have even engraved them on imitations made of say transparent synthetic corundum to make their fakes look real (remember that the number of synthetics you see in the trade is inversely proportional to the distance from the mining area, I have seen some very cunning ones such as pouring green lass into a mould made with an emerald crystal and then sticking gangue minerals on over the result).
Loz
Specimen from South Africa, 1.5 x 1.5 x 0.4 cm, 9.94 carats weight. Image credit: Rob Lavinsky/iRocks.com
