I found this more pale (instead of the darker, richer color most people prefer) amethyst pyramid while I was in Sedona last summer (2013). I went through my favorite crystal healing store with my pup and parents (family vacation), and spotted a set of amethyst pyramids they were selling. Most of them were fairly pricey as pyramids tend to be, but I noticed this one was on a 'discount' shelf. They said it was there because the shipment had taken damage in transit, even though the pyramid I was looking at had minor scrapes at worst. I was actually pushing for a full (or more full) price, because I didn't think there was any way it deserved to be on a damage discount, but they insisted.
Anyway, I digress! This beautiful pyramid has been amazing to have. It holds energy so amazingly well, and because of where I placed it, the sun will cut through the edge of the blinds at a certain time, and connect with the pyramid. If the blinds are closed, the pyramid lights up with a beautiful purple/violet light in the middle of a darker room. (the photo is when the sun started moving away from it already, but you can see what I mean)
It's brought a surprising amount of energy and focus to me, and I couldn't be happier with a purchase like this.
So, it's not a part of an altar, but it's definitely my current favorite.