Relationship of Type to Synthetic Diamonds. Over
the last several years, the production of HPHTgrown
synthetic diamonds has increased dramatically,
and chemical vapor deposition (CVD) synthetic
diamonds have started to enter the gem market
(Wang et al., 2007). Therefore, gemologists are under
even greater pressure to identify these laboratorygrown
products. Diamond type can provide a few
clues in this regard. HPHT-grown synthetic diamonds
are almost all type Ib, a type that is rare in
natural diamonds. Those few natural diamonds that
are type Ib usually contain abundant inclusions of
natural minerals and exhibit colorful strain patterns
(discussed below). In contrast, type Ib HPHT-grown
synthetic diamonds contain only metallic flux inclusions
(when any inclusions at all are present), and
they typically show a very weak strain pattern or
none at all (Shigley et al., 2004). CVD synthetic diamonds
are most commonly type IIa and are typically
near-colorless or light brown. These synthetics can
usually be distinguished from their natural- and
treated-color counterparts by the absence of crosshatched
“tatami” strain patterns (see also below).
Occasionally, both HPHT and CVD growth techniques
will produce attractive blue type IIb synthetic
diamonds. The absence of “tatami” strain, combined
with the presence of electrical conductivity,
can be used to identify these synthetics.