In past issues of Cabbin’ Fever, I have described the interpretation of fabrics in cutting rough. Another consideration, however, is the bounding surface of either your crystal or grain. That is to say, beyond the fabric, the inherent initial shape of your rough (as determined by growth, or even fracturing) may be utilized in guiding the shape of a finished gemstone. Take, for example, the rough obsidian I have been working on for a cutting order.
You can see that when the rough is held to display a relative symmetry, that it has a great width, and a stout, tapering length. Due to this initial shape (and the interests of the customer) we determined that the most interesting, and efficient shape for this piece would be to form a heart shape for a pendant.
Had we made a standardized dome cabochon, a greater amount of time would have been used bringing that down the taper past the center of the cabochons dome. By making a heart shape, we can create a bisymmetric stone which shows off the natural taper of the break, and minimize cutting time (which, given that it is obsidian, it wouldn’t take long anyway). This also reduces material loss, and maintains a greater carat mass.
For any questions, and greater detail, feel free to ask, I will likely get back to you.