After digging around a little- and hoping a Leopold biographer would stop by- I can say only that this is possible. When Leopold ascended to the throne in 1865, aluminum was indeed a very expensive metal. It could only be made by reducing alumina minerals with sodium, and that process ( discovered by Frenchman Henri Sainte-Claire Deville in 1855) was practical for only small amounts- kilos, not the tons later possible with electro-chemical reduction. And it's possible that decades later, after Leopold had acquired his Congo slave empire and began spending lavishly, an aluminum crown was one of the items he bought. But I can't find a specific reference.

However, an aluminum crown really does sound like something that French Emperor Louis Napoleon would have done. Napoleon was quite taken with aluminum, and gave a lot of support to Deville's laboratory. In the Exposition Universale of Paris in 1855, Deville publicly placed an ingot of aluminum in Napoleon's hand, and the Emperor immediately commissioned a baby rattle to be made of it for his son. He would later go on to talk of having curiasses made of aluminum for his cavalry: this would have been very, very expensive, but then Louis Napoleon had a lot of hubris. An aluminum crown seems his style. Leopold , it seems like, would only have had some aluminum jewelry made for one of his mistresses.

Jules Guy:Henri Sainte-Claire Deville, Sa Vie et Ses Travaux 1889