What is "opus vermiculatum"

Opus vermiculatum is a method of laying mosaic tesserae to emphasise an outline around a subject. This can be of one or more rows and may also provide background contrast, eg as a shadow, sometimes with Opus tessellatum. The outline created is often light and offset by a dark background for greater contrast. The name opus vermiculatum literally means "worm-like work", and has been described as one of the most demanding and elaborate forms of mosaic work. Usually opus vermiculatum is meant to put emphasis on the main design and foreground details of a work, using a smooth and flowing halo-effect. Sometimes it was used only around the head of a figure. The tesserae used were often square but can be variously shaped.