Finding it really difficult to identify this stone between diamond and moissanite.
Test results :
Diamond tester thermal - passs
Polariscope - light and dark through 360 anisotropic
Has black service features
Moissanite of any practical size is man-made with the following traits:
Refractive Index: 2.65-2.69 Doubly Refractive
Specific Gravity: 3.22
Diamond has the following traits:
Refractive Index: 2.417 Singly Refractive
Specific Gravity of 3.52
Measure your refractive index.
I do not think the stone as presented is diamond or moisannite.
Based on Specific Gravity alone (and assuming some amount of rounding) your specimen is most likely in the tourmaline group. The second most likely within the amphibole group. There are nine other stand-alone possibilities, none of which approach sufficient commonality for testing until tourmaline and amphibole have been thoroughly ruled out.