In this study, the stress-strain curves of four different rocks are obtained by using true tri-axial test apparatus. First, results using this test apparatus are compared with results using conventional tri-axial test equipment and the dependability is confirmed. Mohr circles, based on the stress-strain curves have been drawn to provide yield envelopes for the rocks tested in this study. The yield envelope is approximated by a bi-linear curve and by extending the wedge indentation theory for this case, the penetration force equation is derived. The calculated values for the penetration force are then compared with the experimental results obtained by previous investigations. The rocks tested are: Danby Marble, Carthage Marble, Ohio Sandstone and Indiana Limestone. These rocks are tested under confining pressures of 25, 5, 75, 1,, 2, and 3, psi.