Whenever Michael Fremer told his friend Larry Marcus, an Ann Arbor hifi dealer, about a new cartridge design, Marcus answered with two words, "Koetsu Coralstone." Fremer protested that it was a conventional cartridge in a stone body, and Marcus said "Right." Inside, each model is a classic moving coil cartridge, where tiny coils ride on the stylus arm inside a magnet. What changes is the body around them. Dick Olsher of The Absolute Sound explains that a stylus picks up vibration much like a seismometer, so each body adds its own flavor, and he believes Yoshiaki Sugano chose aluminum, rosewood, lacquered rosewood and stone partly for the color each one brings. Reviewers keep praising the midrange. Olsher heard a "big sky" midrange from the Rosewood Signature, and Gary Lea of Dagogo called the Urushi Vermillion's midrange "absolute magic." Koetsu suits listeners who care most about voices and acoustic instruments and who listen for hours at a time.

Start with the Black Goldline, the only aluminum-bodied Koetsu. Michael Corsentino of Positive Feedback found it "more neutral than anything else" and suggested the wood-bodied Rosewood for a warmer sound. Paul Rigby of The Audiophile Man heard a very large soundstage from the Black. The step up is the Rosewood Signature, then the Urushi models in lacquered rosewood, then the stone bodies. Lea found the differences among the stone models subtle, apart from the Coralstone, and suggested choosing by looks. Output is very low, so plan on a strong phono stage or a step-up transformer, and take care with loading. Lea settled on 100 ohms and Olsher on 50, so start in that range and listen. The Analogholic channel ran a Black on a Rega Planar 8, while Fremer says the Onyx Platinum needs an arm able to control its bass.

Yoshiaki Sugano boxed in his youth, knew Japanese swords, painting and calligraphy, and spent a long career at Toyota before retiring at 60. He had studied and modified commercial cartridges, and in retirement he began building his own by hand. He named the company after a distant ancestor, the craftsman, potter and calligrapher Honami Koetsu (1558–1637), whose credo Olsher quotes: "nothing so exquisite exists in this world as an object made by the human hand." Sugano went to research labs and specialist craftsmen for parts. An AudioEnz writer describes a small plate around the shank of the diamond that resembles the tsuba, the hand guard of a Japanese sword. Demand grew after The Absolute Sound wrote about Koetsu, and Rigby calls them "the first hand-made cartridges to make a splash in the audio world." Reviewing an Onyx Platinum for Tracking Angle, Fremer closed by quoting Art Dudley, who heard "color, texture, touch, and musical nuance" as never before.