Koetsu cartridges deliver a warm, rich and “saturated” sound without sounding murky or slow sounding which is not an easy trick to pull off.
That’s a perfect description @larryi
I used to think they were stupid, overpriced little boxy rocks. Then I actually heard one, and have been a huge fan ever since. I have 6.
One of the problems was that the North American and Euro distributers would artificially amp up the price of "premium" stones (Coral, Blue Lace) versus "base" stones (Onyx, Jade) to segment the high-end of market. The price disparity was much more modest in other markets (Japan, SE Asia). That said, they were all still expensive cartridges, and the Blue Lace I have does in fact sound better than all other stones. The Coral - eh, maybe not so much. Perhaps my Coral needs more burn in, but the Blue sounded amazing from hour #1. Could be some variation from sample to sample, too.
Supposedly Koetsu bought up the whole run / supply of Platinum magnets, which is why other manufactureres don’t use them (only Lyra, in very limited fashion). They are nice sounding magnets, more smooth and creamy sound versus samarium-cobalt and neodymium. Alnico is pretty neat too, though.