All the currently known geometric microliths share the same fundamental characteristics – only their shapes vary. They were all made from blades or from microblades (nearly always of flint), using the microburin technique (which implies that it is not possible to conserve the remains of the heel or the conchoidal flakes from the blank). The pieces were then finished by a percussive retouching of the edges (generally leaving one side with the natural edge of the blank), giving the piece its definitive polygonal form. For example, in order to make a triangle, two adjacent notches were retouched, leaving free the third edge or "base" (using the terminology of Fortea). They generally have one long axis and concave or convex edges, and it is possible for them to have a gibbosity (hump) or indentations. Triangular microliths may be isosceles, scalene or equilateral. In the case of trapezoid geometric microliths, on the other hand, the notches are not retouched, leaving a portion of the natural edge between them. Trapezoids can be further subdivided into symmetrical, asymmetrical and those with concave edges. Lunate microliths have the least diversity of all and may be either semicircular or segmental.