It is a well-known fact that natural stones have great strength and resistance to various external influences. This is a typical characteristic of the slates that are extracted at a great depth – they are less brittle, do not crumble over time and do not weather, unlike the slates that are extracted from the surface layers.
The gneiss field in the region of Ivaylovgrad is at a great depth, and because of this the extracted rock material has excellent quality.
The table below, showing the technical characteristics of gneiss, supports this fact.
| Parameters | Result of measurement | Standard |
| Strength under pressure when it is dry | 129,3 | 120 – 130 |
| Strength under pressure when soaked with water | 136,1 | 100 – 150 |
| Softening coefficient | 0,96 | Not required |
|
Coefficient of pores |
103,6 | Not required |
| Coefficient of cold-resistance | 0,86 | Над 0,8 |
| Loss of mass after 25 cycles – average value | 0,004 | Not required |
| Appearance of the frozen samples | The appearance does not change | Bulgarian State Standard (BDS) 11485-73:Visual assessment at every fifth cycle |
The rock material, extracted in the Eastern Rhodope Mountains, is strong, has great thermal resistance and excellent parameters as regards wearing out and durability in cases of rapid changes in temperature.