I swear by my old, US military issue folding Army shovel. That thing is SO strong. I've had it for over thirty years.
I actually broke the wooden handle in half about 15 years ago (but the shovel part didn't budge one bit). As it turns out, it's now the perfect length to fit into my small backpack and perfect for ANY digging. I have NEVER encountered a rock that it cannot handle and I always bring it along for tough rocks.
If they're good enough to dig foxholes and trenches anywhere in the world they're more than suitable for metal detecting under any conditions.
Ironically, where I am I'm having the opposite problem over the last few days. We've had lots of rain and I'm running into mud lately.
Oh, one more thing. I also bought a cutting tool made by Ames at home depot. It's very similar (practically a knock-off) to the Fiskars digging knife (which is actually what i was looking for at the time but they didn't carry it and I had already been to two other stores). I don't have the money to blow on something better right now but this thing cuts a plug perfectly and it's really tough. It cost me a whopping 10 bucks.
But again, I prefer my US "D" shaped folding shovel over a spade any day under dry conditions. The newer ones are a bit different than mine, which came with a really hard wooden handle. They have a large, triangular handle at the top of the shaft and I believe they fold in three sections as opposed to two.
Here's a link:
http://www.amazon.com/Military-Original-E-Tool-Entrenching-Shovel/dp/B004SOPJG4?tag=treasurenet01-20
Using a spade is just too much work when it's dry. You need to bring a sledge hammer along to drive the thing into the ground.