In Exodus 28:19, it is not certain what kind of stone this was, but it was azure from white, which signifies spiritual good. It also signifies natural good, or that of the lowest heaven. (Arcana Coelestia 9870, True Christian Religion 609)

An agate, one of the precious stones in Aaron's breastplate, signifies the spiritual love of good (Exodus 28:19).

9870. 'A lapis lazuli, an agate, and an amethyst' means the spiritual love of good. This is clear in a similar way from their colour; for the blue colour which is a product of white means spiritual good, or what amounts to the same thing, the spiritual love of good, see above in 9868. The spiritual love of good is charity towards the neighbour, and the spiritual love of truth is faith rooted in charity. That good and this truth are what constitute the second heaven. The internal part of that heaven consists in the good of charity, and the external in the good of faith. It is well known that a lapis lazuli is of a blue colour, and an amethyst as well. But it is less well known that an agate too is of this colour; for no one knows what kind of stone the word in the original language describes, whether an agate, turquoise, or some other.