To find an answer to the question what makes an object with runes Frisian, runologists turn to its provenance and runological and linguistic features. Quak (1990) gives an overview of these aspects.
Although a find in modern-day Fryslân indicates the Frisian origin of a runic object, two more or less complicating factors have to be taken into consideration. First of all: the boundaries of Frisia in the early Middle Ages are different from the province of Fryslân we know today. The exact boundaries are not known, but the coastal area of the Dutch provinces of Zeeland, North and South Holland, Friesland, Groningen en the German coast of Niedersachsen is generally regarded as Frisian territory in the early Middle Ages. Secondly, finds in the Frisian area could have been brought here (1), which also implies that it’s also possible that finds outside the Frisian area can be Frisian. An example is the Amay comb that - although it was found in Belgium - is considered to be Frisian.
Somewhere before the 6th century, Frisians and Anglo-Saxons developed extra runes to respond to sound-changes that had taken place in their language, often referred to as CoastalGermanic. Two new runes were added to the Germanic futhark of 24 runes. The a-sound (ansuz-rune) developed into an o, which was represented by the so-called os-rune o. It kept its place in the futhark, but since the sound was changed, the Anglo-Frisian version is called the futhork. Consequently, the original o rune for o, got a different value in the futhork: œ. As the a-sound obviously was still used, a new rune was developed for it, which was the a, called ac-rune. The original a-rune was also still in use, but with a different value: æ. Some futhork runes were adjusted: the traditional h got an extra diagonal twig and became an h and the diamond-shaped rune representing the ŋ sound got extra twigs and became a n.
The language spoken in the Greater Frisia of the early Middle ages, was rather undistinguishable from the languages spoken in England and the north of modern day Germany (2). Around the 6th century some features can be recognized that indicate a differentiation of this Coastal Germanic into Old English, Old Saxon and Old Frisian. Most importantly, monophtongization took place in the Westgermanic au, resulting in Old Frisian a. Across the Northsea, a similar innovation took place, resulting in Old English ea. Another linguistic feature is the suffix -u that is thought to be a Frisian form, coming from Gmc -a ending.
1 which is the case with the bracteat from Hitsum (pictured above) with the inscription foRo, which is thought to be a female name of Scandinavian origins.
2 Looijenga, 1997, 35