How many calories are in half oz of drinkable green clay?!

Not enough to worry about... half an ounce is about 15 grams (1 tablespoon) of content and just not enough to be of a major concern (even if it were sugar or fat, it wouldn't be too much). However, clay is more like fiber in that it has no nutritional value, but it helps to absorb things and remove them from your system... as a cleanser, your body doesn't absorb it or break it down to get nutrients out of it.

If you're buying it in a store, there would have to be a "Supplement Facts" label on it that would outline the nutritional benefits as well. The guidelines set up on RDAs (Recommended Daily Allowances) is that they must be listed on the label if they're above certain levels. The calorie level is 5 so if it had more than 5 calories per serving, it would have to be listed on the label for how many calories it has... if it doesn't list it, it's less than 5.
I assume it doesn't list it and that's why you're asking, so it has less than 5 calories. ;-) I hope I helped!

Health/nutrition classes and personal research in my role as a Product Specialist for a vitamin company (we get asked similar questions quite often)

You are not a plant. None