1
Striations betray the twin
Parallel grooves run at right angles across adjacent faces — the surest field sign that {100} and {210} forms have intergrown. faces {210}
2
Brass is luster, not colour
Record the metallic sheen and any tarnish iridescence; the pale brass-yellow shifts under raking light where flat yellow never would. H 6–6.5
3
The streak tells no lies
Drawn on porcelain it comes greenish-black — the one test that separates pyrite from the gold collectors hope for. SG 5.01
4
Matrix sets the scale
Note the slate-grey host rock beneath each crystal; the dark ground is what lets the brassy faces read as figure, not noise. matrix · slate
FIG. 7
Pyritohedron, twinned