These cups are high fired slip cast porcelain and built to last. The glazes are food safe and dishwasher safe. If you grow bonsai, you might be tempted to drill a hole in the bottom and put a nice Chojubai in it, or maybe a small Kaede. Just remember, you will not be able to drink your cappuccino out of it anymore.
The actual color and crystals formation may vary from cup to cup. The pictures here represent the range and difference.