What's "W"?

"These are glass coasters for the science person. The Periodic Table isn't quite square, so I used my artistic license to enlarge the bottom bit for the bottom two coasters. These ones are made with French vanilla glass on top of dark purple, turquoise, orange and lime green glasses. The table is fused into the glass permanently. Rubber bumpers are attached to protect furniture.

Each coaster measures 3.25" square."

Found at PollyFusia's Etsy shop.