As mentioned in Quartz: Part I, quartz contains two main groups of gemstones – macrocrystallines which include transparent gems like amethysts and citrines and microcrystallines which include translucent and opaque gemstones like chalcedony. Agates, carnelian, sard, chrysopase, jasper and bloodstone are varieties of chalcedony (although it should be noted that the latter two stones are sometimes considered to fall into another category of microcrystalline). These gems were commonly used in antique jewellery in seals, intaglios, watch keys and signets rings. Agates and jaspers in particular were used in Victorian Scottish ‘pebble’ jewellery. Mourning jewellery also used a lot of onyx.
There is often some confusion when we talk about microcrystallines used in jewellery. One area of confusion is what we mean by ‘chalcedony’. The main category of microcrystallines is chalcedony but there are also specific varieties of chalcedony within the category which are referred to with a specific descriptor, such as white chalcedony which used in the brooch below. There are also colourless, blue and grey chalcedony.
Another area of confusion is that of banding. Agate is a variety of chalcedony and as a group is known for its concentric banding. However we talk (and I know I have done this) of banded agate when really we know that agate is banded so it is really unnecessary to specify that the piece is banded.
The bands of different colours in agate, usually creams, blacks, whites, grays and browns, can be used to great effect in jewellery as seen in the drop earrings above. And that banding also led to agate being used for cameos. Agates with parallel bands were chosen, normally with a light layer of banding above a dark layer, although sometimes three different coloured layers were used.
Having said that agate is generally banded, there are some examples of agates without clear banding. These are specially named types of agate, such as moss agate, also known as dentritic agate and landscape agate. They contain internal inclusions resembling plants, trees, and mountains. The photo below is of a piece of moss agate. It has a translucent white background plus green inclusions that really look like moss.
If you want to know more about agates, the website below has some lovely photos and interesting articles.