This may be a really stupid question but......
found some old sets in a second hand and oriental shop. Believe they were bought at auction in Japan along with gobans and japanese furniture for re sale here in NZ. The markings clearly show that white are shell but how can I tell if the black are slate. They don't look or feel any different to glass stones I have.

I own several sets of stones. The black stones from my glass sets also look identical to the slate stones from my shell & slate set (all are somewhat matte).

I have also seen really reflective black stones that were unmistakably glass before. But those I have I can only tell which belong to which set from their slightly different sizes.

So I'm not sure if they are really glass.

If the white are shell then it's a good bet that your black stones are slate.

Take one of the stones and wash it very thoroughly so all oil and dirt is removed. If the stone is matte colored and has a mildly rough surface then it's probably slate.

It's not uncommon for someone to slip in a glass stone or two to replace any lost or broken stones, so look for anything different.

Ask the proprietor if you can bring your glass stones to the shop for comparison. My glass stones are easily discernible from my slate stones. I support the idea that it is unlikely the slate stones have been swapped out for glass. There is no good reason to do that. The shells are more valuable in the craft markets.

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David Bogie, Boise ID
I play go, I ride a recumbent, of course I use Macintosh.