Possibly the most unusual pseudomorph (replacement) that one can imagine: talc, the softest mineral, has replaced quartz, one of the hardest minerals on the Mohs scale! This matrix specimen exhibits a single, isolated and exceptionally well formed quartz crystal, 2 cm in length, that has been totally altered to an ivory-colored talc. Most specimens are jumbly messes or have extensive damage, breaks, or rubs to the (former) quartz. Most are almost massive, with small tips pocking out. This one is a dramatic and aesthetic example that just stands out. I love these things and buy every good one I can get hands on. As they came out so long ago (I am told the late 1800s and early 1900s), this is generally only when very large old collections turn up on the market again. Overall, these are perhaps my favorites among pseudomorphs.