The tilma is made from what is commonly called “cactus cloth.” Cactus cloth is made from fibers within the agave plant. The typical lifespan of cactus cloth is about 30-40 years, and then it disintegrates. Yet the tilma has lasted for 472 years!
In 1921 an anti-Catholic placed a vase of flowers in front of the tilma. Inside the vase was a stick of dynamite. The explosion bent an iron cross that was several feet away, but didn’t damage the cloth in any way.
There’s no rational explanation as to why this primitive cloth has not deteriorated, other than a miracle.
In the image, the garment worn by Our Lady of Guadalupe is a bluish-green robe dotted with stars. The placement of these stars corresponds to a map of the constellations, as seen from Mexico City, in December 1531, when the image was created.
Keep in mind, this is not a painting of Our Lady, as St. Juan Diego remembered seeing her, this is an image that miraculously appeared on his tilma. It is, in effect, a self-portrait left by the Blessed Mother.
The miraculous image is actually a secondary miracle. In the first miracle, Juan Diego was told to climb a hill and pick some roses. Since it was December, he did not expect any to be blooming. Not only were the roses in full bloom, they were a type that at the time grew only in Spain. Our Lady placed the roses in his tilma, and when he spread them out for the bishop to see, that is when the image of Our lady of Guadalupe was revealed.