Greenockite has the form of a regular six-sided prism, with pyramidal summits, the faces of the pyramid being inclined 36° 20′ to their base. The pyramids are sometimes truncated on their summit.

The crystallization is often composite.

The index of refraction of Greenockite is 2.6882, corresponding to the middle of the green space, and to the ordinary ray. Hence Greenockite exceeds the Diamond in refractive power, and also chromate of lead, which I had found to surpass the diamond in this respect.

The double refraction of Greenockite is comparatively small, which is not usual in substances of a high refractive power. It is so small, indeed, that owing to its great dispersive power it is exceedingly difficult to separate the two images.