Over on reddit, u/utarohashimoto put up a link to Songyun's $30 ceramic weiqi stones. Made in China, these seem to compete directly with Yunzi's proprietary formula.
https://www.reddit.com/r/baduk/comments ... mic_stone/
They're available in the States from amazon: https://www.amazon.com/gp/product/B07THWQ64Z
Puffery: "Songyun is not only a well-known and largest Go brand in China, but also a comprehensive enterprise of Go culture and education communication. The company covers an area of 15,000 square meters, have sales, production, warehousing, shipping, research and development departments, has nearly 50 patents and inventions, and products are sold in North America, Europe, South America and other regions.
In November 2009, US President Barack Obama visited China, and the agate Weiqi stones in the national gift presented to China came from Songyun."