Gemcutter here, with a background in synthetic gems and crystal chemistry (am actually in medicine but I'm a rock nerd lol). What you're asking for isn't possible, as one of the commenters has noted.

But, something else is. There are purple sapphires, which have a very similar colour to amethyst and even have similar pleochroism to amethyst. There are also blue lab-grown sapphires. One manufacturer is currently growing synthetic sapphires that have alternating bands of blue and purple.

If you're not particularly attached to the idea of amethyst, and are okay with amethyst-coloured sapphire instead, you could get one of these pieces.