Cobalt blue is one of the most lightfast of the blue pigments, and from that perspective is not adequately replaced by mixtures of phthalocyanine blue and synthetic ultramarine, neither of which are as lightfast.
The natural Lapis has an ancient history, Ultramarine was first synthesized in 1828;
— the Natural mineral form of synthetic PB29. It has always been and still is a very expensive pigment. Extraction from the rare semi-precious gem stone is labor intensive & time consuming;
* Although Lapis Lazuli (after purifying) and Ultramarine Blue are essentially the same chemically, Lapis Lazuli is the natural mineral source and has different working properties, it also contains iron pyrite crystals that can give it a slight ‘sparkle’. Lapis Lazuli is also a much more expensive pigment so it is misleading to label Synthetic PB29 as Lapis Lazuli.