I'm aware that melting changes the structure, and I didn't say it would keep the color, I even suggested that it wouldn't have the gradient op was looking for and would be more of a seam than a mixture. I was imagining exactly what you described: two crystals joined by an amorphous obsidian like glass.

What if you took two large gemstones, let's say a sapphire and an Amethyst. Shaved a flat side onto each, then instead of melting the whole thing, you very quickly applied a massive amount of heat only to the flat side. Then as soon as the edge begins to melt, you stick the two flat sides together and let them glue together. The seam probably wouldn't be a pretty gradient like OP wanted, but it might still work? Idk.