I needed a quick pair of dividers to gauge the distance from either side of a face-drilled hole on pendants. About 20 years ago I bought a 4 foot hunk of braided 1-inch copper cable. We have been cutting off of that cable for various things since then. This tool used that cable, or a single strand of it. Semi-neat bending with pliers and a beveled sharpening to opposing points gave a quick to adjust inexpensive measuring gauge. The hardness of different parts of the stone as well as the contrast in color of those different parts of the stone can make your perception of symmetry a little skewed as well as making it more difficult to grind the sides equally. Gauges are great for getting you close but for the last little bit, the eye is always best. Sometimes the girdle of the stone is not perpendicular to the faces. The reason being that if the hole is slightly misdrilled, that is, at an angle, or the decorative elements of the stone around the hole are very different from one side to the other, the shape may have to be adjusted a slight bit. Tilting the girdle allows you a bit of leeway from one side relative to the other. You just have to remember to sand the girdle as made and not perpendicular.