Very big question. The best lap material depends on your technique, what material you’re cutting, what you are using as a cutting/polishing media, and what the operation is. About the only purely mechanical property is that it be flat with parallel faces.
If you have specific questions, you will get specific answers, usually several different ones. We don’t always agree on the best way to do things. 🙂
Otherwise, research. Search the message boards at Gemology Online, or the old USFG Faceter’s List on Yahoo. You’ll find information on commercial sites, e.g. http://gearloose.co/product-charts/ (though the charts there are not updated with the latest laps.)
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