  Hexagonal diamond has been predicted computationally to display extraordinary
physical properties including a hardness that exceeds cubic diamond. However, a
recent electron microscopy study has shown that so-called hexagonal diamond
samples are in fact not discrete materials but faulted and twinned cubic
diamond. We now provide a quantitative analysis of cubic and hexagonal stacking
in diamond samples by analysing X-ray diffraction data with the DIFFaX software
package. The highest fractions of hexagonal stacking we find in materials which
were previously referred to as hexagonal diamond are below 60%. The remainder
of the stacking sequences are cubic. We show that the cubic and hexagonal
sequences are interlaced in a complex way and that naturally occurring
Lonsdaleite is not a simple phase mixture of cubic and hexagonal diamond.
Instead, it is structurally best described as stacking disordered diamond. The
future experimental challenge will be to prepare diamond samples beyond 60%
hexagonality and towards the so far elusive 'perfect' hexagonal diamond.
