During the middle ages, it was tradition for knights such as kings, emperors, princes and nobles to carry their valuables into the battle field. The valuables include, diamonds, precious stones and most importantly Pearls. These were called their 'Personal Ornaments'. Pearls were fixed onto their battle uniform as they had a strong belief that by doing so it would protect them and keep them safe. 
As they wore their valuables in the battlefield for protection, most of these valuables were lost or destroyed on the battlefield. Which can explain why only a small portion personal ornaments of the monarchy of this period have been preserved up to today. 
Pearls in the Renaissance period were so highly regarded that several European countries passed laws forbidding anyone outside the nobility from wearing them. 
Photo: History.com - Chivalry knights Middle Ages painting