I use quite a few different things. Always have a stone and sandpaper. Using different grit materials will make the call sound different and make it more versatile. I will say that one of my favorite materials to condition crystal and glass is the roll of heavy duty sand paper from O'Reilly's autoparts. It's about 3/4" thick and about 24" long. I cut it off in 1" sections. It's 80 grit and/or 100 grit. That little piece can be used multiple times and I love the sound I get from it's conditioning. Always wipe the dust from the crystal, so it doesn't clog the grooves in the conditioning and/or clog the striker faster.
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