Collapsed building firm Wrekin Construction owed nearly £21 million to creditors when it fell into administration - but the true value of its cricket ball-sized ruby remains shrouded in mystery.

According to a creditors’ report seen by the Birmingham Post, the valuation of the so-called Gem of Tanzania - a 2kg ruby said by the firm to be worth £11 million - was based on a forged document, the source of which is still not known.

The report also details the names of over 1,000 unsecured creditors owed money by Wrekin Construction, including Birmingham City Council, which is owed £1,114 and Staffordshire County Council, which is owed a total of £13,862.

Wrekin Construction was taken into administration in March with the loss of nearly 500 jobs after winding up petitions worth millions of pounds were filed by unpaid contractors.

Copies of the firm’s accounts from Companies House revealed how a year earlier the firm had issued £11 million worth of shares to Tamar Group, a vehicle owned by David Unwin, in exchange for a ruby named the Gem of Tanzania.

The £11m asset helped boost Wrekin’s balance sheet from an £8 million liability  at 31 March 2007 to net assets of £6 million.  But in the creditors’ report Wrekin’s administrators Ernst and Young confirmed that the valuation, purportedly issued by the Instituto (sic) Gemmologico Italiano, is a forged document.

The mysterious ruby was delivered to Ernst & Young on March 20th from Saudi Arabia where it was being held by an agent of Mr Unwin.

But the administrators said they could not reveal the value uncut stone, which measures 12.3 cm by 10.8 cm by 6.5 cm and weighs 2.14kg. The report states: “At this time it is not possible to place a value on the uncut stone given its unique nature and consequent absence of comparable market data upon which to base any estimate.

“Furthermore, any attempt to provide a potential range of valuations may prejudice future realisations.”

Gemmologists consulted by the administrators said the gem was of an opaque nature but its crystal structure meant that it is not thought to be of sufficient gem quality for faceting - but it may be suitable for what is known as cabochon - being shaped and polished as opposed to faceted.