Mariano Fortuny was a Spanish designer who studied achlemy, art, science, music, architecture and history all over the world with a particular emphasis on Greek and Egyptian history. Oddly, he also had an obsession with Wagner's music. When he started his foray into fashion in 1907 he decided to open his own fabric production so that he could have access to the high-quality fabrics he envisioned his designs to be made of. It even seems that neither World War managed to completely shut down his factory, either.
The processes that were and still are used to create these fabrics is still a trade secret and the Fortuny company still claims that no one has been able to exactly replicate their fabrics. The factory is still using the same machinery and processes that were implemented by Mr. Fortuny himself. Some of the processes that are known to be used are dying the fabric from the back and the use of dye chemicals from achlemy. Regardless of what secret thing they do to make their fabric as wonderful as it is, what really makes Fortuny fabric really so wonderful is that it truly stands the test of time. I'm inclined to say that no other designer seems to have as many surviving gowns from the first few decades of the 20th century as Fortuny and I think that is the true testament to the quality of his work.
Picture of Fortuny gown from http://americanhistory.si.edu/collections/hispanicdesigners