Ask-a-Geologist #19: What Gives Amethyst that Pretty Purple Color?

What makes amethyst purple?

Lily wrote to Mini Me Geology and asked about the mineral amethyst. Specifically, she wants to know what gives amethyst that beautiful purple color. In this video, we talk about why amethyst is purple, its relation to quartz, and an interesting fact about how citrine fits into that mineral family.

Well Lily, you’re correct. Amethyst, this sample here, has a beautiful purple color. Sometimes you’ll find a lighter purple, sometimes you’ll find actually darker purple colors. What you might not know, is that amethyst is actually a form of the mineral, quartz.

Purple Amethyst Sample

Amethyst is a Variety of the Mineral Quartz

Quartz is a clear mineral and it has the same crystal shape as the amethyst. These two minerals share other properties like hardness and luster as well. All of the basic properties are going to be the same. The chemical formula for both minerals is SiO2, silicon dioxide. The difference, and what gives the purple color, is that the amethyst crystal has ferric iron in the chemical formula of the mineral itself, which changes the color from clear to purple.

Clear Quartz Sample

There is something else that you might be interested in knowing about amethyst. That is there is another related mineral called citrine. In the video, the citrine is in a druze, where a bunch of smaller crystals sits side-by-side. However, the individual citrine crystals have the same pointed shape on top as the amethyst crystals. Citrine actually forms from an amethyst crystal when it’s heated. That this can happen in two ways.

Golden-Brown Citrine Sample

One, it can happen in nature if the amethyst is in an area that has naturally high heat over time, the crystal is going to change from a purple to a brownish-golden color, and then it can also happen in the lab. If some people who are creating gemstones for jewelry or other items, want to create a nice dark citrine, they will take an amethyst and heat it with super high heat and that will actually change it to a really dark golden-brown color, and you see a lot of those in jewelry. So, these citrine and amethyst are very closely related and they are both forms of the mineral quartz.

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