It is a mineral rock which provides the most intense deep blue pigment. When it began to be imported into Europe in the Middle Ages, blue was a difficult and expensive colour for artists to obtain and it became the basis of French Ultramarine paint for centuries until artificial pigment could be manufactured. This had the effect of making it rare and costly so that it became a status symbol in art in much the same way as gold leaf.
|The Madonna, Sassoferrato, 17th century|
It is useful to bear in mind that this rule, as with all symbolism, should not be seen as immutable; artists were free to create alternative values but the 'meaning' of a picture would often need to be 'read' through it's depictions and colours.