Jewels of the Crown: Historical mystery by Robert Henricks

The year is 1216. Since assuming the position of the king's wagon master, Norman Gervase has been tasked with safeguarding the royal baggage train, an obligation that places the welfare of the crown jewels squarely in his hands. But news of the king's death complicates his mission and draws him into a frantic race to salvage the king's treasure.
Having no alternative, he turns to a trusted soldier with a purpose: to seek out a safe haven for his priceless charge, a hoard of inestimable bounty in gems, and to keep their anonymous whereabouts an utter and remote secret. But such a resolution compels him to make the agonizing decision as to who he can trust among soldiers, royal officials, clergymen and deserters he meets along the way en route to establishing a mysterious organization which pledges to safeguard the crown jewels in perpetuity. In this story, the thirteenth century becomes entwined with a modern day clash of interests for the chance to possess King John's trove of incalculable wealth that has remained hidden for eight hundred years.