Study of fracture mechanics in heterogeneous rocks, including crack initiation and propagation, has practical applications for geocontrol and identification of fracture zones in hydrocarbon well extraction. The features of microcrack zone formation depending on the type of rock heterogeneity under triaxial stress state conditions are considered. The research was conducted using an MTS 815 servo-hydraulic testing frame integrated with a Milne DAQ acoustic emission system (Itasca International Company, UK). The paper presents the fracture results of samples of various lithological types manufactured from cores of post-magmatic rocks. Fine-grained samples with gneissic and banded textures, as well as a coarse-grained sample with massive texture, were tested. During the tests, acoustic emission (AE) was recorded using 12 piezoceramic sensors. To describe the geometry of the fracture zone, the coordinates of AE event hypocenters were calculated, then the configuration of hypocenters distribution was analyzed using a tomography procedure (layer-by-layer construction of AE event density maps), and the angles β between the direction of the macrocrack and the axial stress σ₁ were determined. To interpret the failure evolution, trends of the b-factor and AE activity were calculated, the intervals of critical behavior of which were correlated with the localization and tomography data. After testing the samples, the types of their deformation and the mechanism of destruction with the phenomenon of dilatation were revealed. It is established that for the considered types of sample inhomogeneities, various microcrack structures are formed under the same volumetric loading conditions. In fine-grained rock of gneissic texture, a linear distribution of AE hypocenters is formed, indicating the formation of microcracks along the direction of layering. For the fine-grained rock with banded texture, the hypocenter distribution is characterized by the formation of distinct clusters, reflecting areas of the most intensive fracturing. In the coarse-grained massive texture sample, a volumetric distribution of hypocenters is observed, manifesting dilatancy properties and the formation of an extensive microcrack network.