Rock Ice Day

What Is Rock Ice Day?

June 9 is "Rock Ice Day" each year. It was established by Kokubo Seihyo Reizo Co., Ltd., headquartered in Yachiyo City, Chiba Prefecture, based on a wordplay in which 6 and 9 read as "ro-kku" (rock), and was certified and registered by the Japan Anniversary Association in 2018. The goal is to raise awareness of "Rock Ice," a brand of packaged ice, and to highlight its many uses and value.

What Is "Rock Ice"?

Rock Ice is a product name of Kokubo Seihyo Reizo Co., Ltd., and refers to bags of "kachiwari" ice made by crushing large blocks of ice. The idea is said to have come from seeing bagged ice sold at a supermarket in Hawaii. Since its launch in October 1973, it has been used in homes, restaurants, events, and many other settings.

• Launch: Sales under the "Rock Ice" trademark began in October 1973.
• Trademark: "Rock Ice®" is a registered trademark used as the product name.
• Uses: Commonly used to chill drinks, for outdoor activities, home parties, and more.

The Background Behind Rock Ice Day

The numbers 6 and 9 combine to sound like "rokku" ("rock"), and this wordplay became the basis for the anniversary — an opportunity to raise awareness of Rock Ice itself while also highlighting the broader value of ice as something that "cools."

The Social Role of Rock Ice

Rock Ice has become a familiar product that can easily be bought at convenience stores and supermarkets. It is said to be frozen slowly over time to make clear ice that doesn't spoil the taste of drinks as it melts. Besides 1 kg and 2 kg bags, the lineup includes cup-type ice and "The Ita-gori," a slab of ice handy for keeping cooler boxes cold.

Ways to Enjoy Rock Ice Day

• Look into the differences between Rock Ice and other types of ice, such as crushed ice or ice-tray ice.
• Explore the science behind why ice is so effective at cooling (latent heat of fusion, specific heat).
• Research how Rock Ice is used in Japanese households and dining culture.
• Compare ice culture and ice-based dishes and drinks across different regions of Japan.

The Everyday Value of "Cooling Things Down"

June 9's Rock Ice Day is a chance to pay attention to something we use casually every day — ice — and learn about the value and history behind it. Since the 1970s, Rock Ice has steadily woven itself into our daily lives, expanding the ways we use it. This anniversary is a good opportunity to reconsider the role that ice itself plays in our lives.