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I've worn a deep pocket in the bottom of the mortar (the cup), and the edges of the pestle (the striking stone) have started to chip and wear away. This often results in some of the dark grey granite mixing in with the mineral pigment when I am grinding an especially hard mineral, like the lovely yellow Mookaite Jasper I prepared today.
I normally use water to separate out some of the granite so the color of the mineral sample isn't affected by the inclusion of dark grey granite. It's a very inefficient process: I usually end up with a fairly clean sample of larger grains, and another mixed color that includes fine particles of both minerals.
Which brings me to today's discovery. After drying out the fresh sample, I compared it to a previous specimen of yellow Mookaite and found it matched perfectly. This is a rarity in itself: even two stones of the same mineral are very seldom the exact same color.
I combined both samples in a larger container, and when I began to brush out the remaining material from the original container I noticed some material clinging to the bottom, attracted by the magnet I had glued there. It was the granite from the mortar and pestle, which had previously dulled the bright yellow of the Mookaite. The magnet had separated the two mineral samples far more effectively than my water sifting method ever could.
I quickly placed small magnets in the samples that are still immersed in water: very fine particles can sometimes take days to settle enough to pour off the water and dry the mineral powder. I'll see how well this works to separate out the granite dust from the mineral samples, but I'm excited to learn the granite is magnetically reactive, as this will help me grind pure color samples more easily (except with a few other minerals that I've noticed also have magnetic properties).
In the meantime, I have completely resurfaced my beloved Stone-Age tool, using a Dremel with a sanding attachment, and a top coat of stone sealer to limit future contamination of my pigments.