First pieces of gold were found in a farm field by an amateur metal detector who lives alone on disability benefit

A harvest of Anglo-Saxon gold and silver so beautiful it brought tears to the eyes of one expert, has poured out of a Staffordshire field - the largest hoard of gold from the period ever found.

The weapons and helmet decorations, coins and Christian crosses amount to more than 1500 pieces, with hundreds still embedded in blocks of soil. It adds up to 5kg of gold – three times the amount found in the famous Sutton Hoo ship burial in 1939 – and 2.5kg of silver, and may be the swag from a spectacularly successful raiding party of warlike Mercians, some time around AD700.

The first scraps of gold were found in July in a farm field by Terry Herbert, an amateur metal detector who lives alone in a council flat on disability benefit, who had never before found anything more valuable than a nice rare piece of Roman horse harness. The last pieces were removed from the earth by a small army of archaeologists a fortnight ago.

Herbert could be sharing a reward of at least £1m, possibly many times that, with the landowner, as local museums campaign to raise funds to keep the treasure in the county where it was found.

Leslie Webster, former keeper of the department of prehistory at the British Museum, who led the team of experts and has spent months poring over metalwork, described the hoard as "absolutely the equivalent of finding a new Lindisfarne Gospels or Book of Kells".

"This is going to alter our perceptions of Anglo-Saxon England as radically, if not more so, as the Sutton Hoo discoveries," she predicted.

The gold includes spectacular gem studded pieces decorated with tiny interlaced beasts, which were originally the ornamentation for Anglo-Saxon swords of princely quality: the experts would judge one a spectacular discovery, but the field has yielded 84 pommel caps and 71 hilt collars, a find without precedent.

The hoard has just officially been declared treasure by a coroner's inquest, allowing the find which has occupied every waking hour of a small army of experts to be made public at Birmingham City Museum, where all the pieces have been brought for safe keeping and study.

The find site is not being revealed, in case the ground still holds more surprises, even though archaeologists have now pored over every inch of it without finding any trace of a grave, a building or a hiding place.

The field is now under grass, but had been ploughed deeper than usual last year by the farmer, which the experts assume brought the pieces closer to the surface. Herbert reported it as he has many previous small discoveries to Duncan Slarke, the local officer for the portable antiquities scheme, which encourages metal detectorists to report all their archaeological finds. Slarke recalled: "Nothing could have prepared me for that. I saw boxes full of gold, items exhibiting the very finest Anglo-Saxon workmanship. It was breathtaking."

As archaeologists poured into the field, along with experts including a crack metal detecting scheme from the Home Office who normally work on crime scene forensics, Herbert brought one friend sworn to secrecy to watch, but otherwise managed not to breath a word to anyone – even the fellow members of his metal detecting society when they boasted of their own latest finds.

None of the experts, including a flying squad from the British Museum shuttling between London and Birmingham, has seen anything like it in their lives: not just the quantity, but the dazzling quality of the pieces have left them groping for superlatives.

They are still arguing about the date some of the pieces were made, the date they went into the ground, and the significance of most seemingly wrenched off objects they originally decorated. There are three Christian crosses, but they were folded up as casually as shirt collars. A strip of gold with a biblical inscription was also folded in half: it reads, in occasionally misspelled Latin, "Rise up O Lord, and may thy enemies be dispersed and those who hate the be driven from thy face."

Kevin Leahy, an expert on Anglo-Saxon metal who originally trained as a foundry engineer, and who comes from Burton-on-Trent, has been cataloguing the find and describes the craftsmanship as "consummate", but the make up of the hoard as unbalanced.

"There is absolutely nothing feminine. There are no dress fittings, brooches or pendants. These are the gold objects most commonly found from the Anglo-Saxon ere. The vast majority of items in the hoard are martial - war gear, especially sword fittings."

If the date of between AD650 and AD750 is correct, it is too early to blame the Vikings, and just too early for the most famous local leader, Offa of Offa's Dyke fame.

Leahy said he was not surprised at the find being in Staffordshire, the heartland of the "militarily aggressive and expansionist" 7th century kings of Mercia including Penda, Wulfhere and Æthelred. "This material could have been collected by any of these during their wars with Northumbria and East Anglia, or by someone whose name is lost to history. Here we are seeing history confirmed before our eyes."

Deb Klemperer, head of local history collections at the Potteries museum, and an expert on Saxon Staffordshire pottery, said: "My first view of the hoard brought tears to my eyes – the Dark Ages in Staffordshire have never looked so bright nor so beautiful."

The most important pieces will be on display at Birmingham Museum and Art Gallery from tomorrow until Tuesday October 13, and will then go to the British Museum for valuation – a process which will involve another marathon collaboration between experts. Their best guess today is "millions".

Leahy, who still has hundreds of items to add to his catalogue, has in the past excavated several Anglo-Saxon sites including a large cemetery of clay pots full of cremated bone. He said: "After all those urns I think I deserve the Staffordshire find."

Mysteries of Mercia

It is no longer politically correct to refer to the period as the dark ages – but Anglo-Saxon England remains a shadowy place, with contradictory and confusing sources and archaeology. Yet out of it came much that is familiar in modern Britain, including its laws, its parish boundaries, a language that came to dominate the world, as well as metalwork and manuscript illumination of dazzling intricacy and beauty.

Mercia was one of Britain's largest and most aggressive kingdoms, stretching from the Humber to London, its kings and chieftains mounting short but ferocious wars against all their neighbours, and against one another: primogeniture had to wait for the Normans, so it was rare for a king to reign unchallenged and die in his bed.

They were nominally Christian by the date of the Staffordshire hoard, but sources including the Venerable Bede suggest that their faith was based more on opportune alliances than fervour.

In south Staffordshire, at the heart of the kingdom, Tamworth was becoming the administrative capital and Lichfield the religious centre as the cult grew around the shrine of Saint Chad. There were few other towns, and most villages were still small settlements of a few dozen thatched buildings. Travel, if essential, would have been easier by boat: archaeology suggests that much of the Roman road network was decaying, and in many places scrub and forest was taking back land which had been farmed for centuries.

The metalwork in the hoards came from a world very remote from the lives of most people, in mud and wattle huts under thatched roofs, living by farming, hunting, fishing, almost self-sufficient with their own weavers, potters and leather workers, needing to produce only enough surplus to pay dues to the land owner. A failing harvest would have been a far greater disaster than a battle lost or the death of one king and the rise of another.

The world of their nobles is vividly evoked in poems like Beowulf, probably transcribed long after they became familiar as fireside recitations, of summer warfare and winter feasting in the beer hall, where generous gift giving was as important as wealth.

Rich and poor lived in the incomprehensible shadow of a vanished civilisation, the broken cement and stone teeth of Roman ruins studding the countryside, often regarded with dread and explained as the work of giants or sorcerers. One poem in Old English evokes the eerie ruins of a bathing place, possibly Bath itself: "death took all the brave men away, their places of war became deserted places, the city decayed."