|Workshop of François Clouet via Wikimedia Commons|
While Jeanne actually died of natural causes, Catherine was not unhappy about her passing. The two had been locked in a deadly religious dispute--that erupted into three wars--for many years. As the reigning Queen of Navarre, Jeanne wielded a lot of influence and power. When she converted to Calvinism in 1560, she set herself up as a leader in the Huguenot, or French Protestant, movement, in direct opposition to Catherine, who as Regent of France for three of her sons in a row, championed the Catholic cause. In a century known for female rulers (Mary I and Elizabeth I in England and their cousin Mary Queen of Scots, for instance), Catherine and Jeanne dominated the political landscape on the continent. For Catherine, Jeanne was a problem.
But, she always had been. When she was just 12, she refused to marry Anne of Cleves brother. She was beaten into submission, but ultimately got her way when the marriage was annulled four years later because it was never consummated. Instead, she married Prince Antoine de Bourbon. She loved him passionately, despite his philandering ways. Together, the had five children although only two survived to adulthood.
Her loving marriage, however, was also torn apart by religion. Antoine had shared her Protestant sympathies but ultimately decided to remain loyal to the French crown and remain Catholic. When she fled Paris to return to Navarre, he attempted to have her captured and imprisoned (she had not prevented the Huguenots from violently sacking one of his towns), but she evaded the attempt. Nevertheless, she tried to return to Antoine when he was wounded in the siege of Rouen later that year. He died before she could reach him.
So she carried on in the religious wars, never wavering in her Calvinist beliefs. By 1570, she helped prepare the Peace of St. Germain-en-Laye, which ended the most recent war and granted more rights to Protestants in France. It also included a marriage between Jeanne's son Henry and Catherine's daughter Marguerite. Alas, Jeanne did not live to see the wedding. She died of turbuculosis (not poisoned gloves) shortly before.
The peace did not last long. In the St. Bartholomew's Day Massacre, which followed the wedding, numerous Protestants were killed and even Henry barely escaped. Twenty years later, it was Henry who ultimately ended the religious wars but he did it in a way his mother would not have approved: he converted to Catholicism, famously saying, "Paris is worth a mass." His mother, however, would have been proud of his next move: he issued the Edict of Nantes declaring religious tolerance for Protestants.
For more about Jeanne, I recommend the following articles, both of which I consulted for this post:
Jeanne d'Albret - Jeanne of Navarre on About.com's Women's History
Jeanne d'Albret's Penalities on Christianity.com