Well first of all thats after the time skip, which means it's not necessarily the case when he fought Lee... since back then nobody mentioned something like that.
Secondly Deidara talks about this Sand having more chakra infused, which makes it faster and more powerful... not about the sand being infused with extremely hard minerals like in case of his fight against Kimimaro. Imho that stuff with the minerals only applies to the Shukaku Shield, which has much more mass then Gaara's gourd and thus obviously needs to be created from an outside sand source.
The way I understand it Gaara was digging up and mixing those minerals into his sand while he was producing the new sand to fight Kimimaro, otherwise he wouldn't have mentioned it... If this was always the case its more likely he would have said something like "My sand is reinforced with the strongest and most dense minerals" instead.