Slippery kitchen floor (tiled)

My kitchen floor is made up of (stone? composite?) 'slabs', each about 1 foot square.
Unless wearing rubber soled footware, these tiles are VERY slippery, even when dry.
Short of replacing them or roughing them up somehow, what can be done? For example, is there any hard wearing substance that can be sprayed onto the tiles to give them a better grip?
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