Texture of the outer surface is actually not that important, as the longest part of the vulva is more receptive for varying movements and specifically for diameter-changing stimulation, less for "a little knob more or less".
A smooth but fluted / grooved toy (preferably like a direct linked chain of small anal beads connected by shafts, not rope) - even if small in size - can together with your fingers give ways more pleasure than any non-electric toy alone (and even more than usual vibrating toys for humans).
I personally suggest one made from either high quality medical grade silicone - very easy to clean and it keeps the warmth of hot water for several minutes (electropolished stainless steel one is a very durable and easy cleanable alternative but hard to find nowadays).
Doesn't have to be in any animal phallus shape, as those tend to be ways more expensive. The changing diameters have a bigger effect than some small rough dimples and such on the surface, which tend to over-stimulate her quite easily (less enjoyable repeatedly).
The tip from
@Labraguy5 regarding "no cheap material" is absolutely right: don't use plastic ones, either won't stay intact for long or have a chance on toxins releasing from cheap quality material. Or, even worse: not well cleanable and sticking instead of gliding, not a good thing.