The most common causes are over/under-watering, too high a level of soluble salts in the soil (plasmolysis aka fertilizer burn), too much sun @ midday. The 'over-watering' issue could be from a collapsing or poor soil, even if you think you're not over-watering. When you're growing in a shallow pot, even a soil that supports only a half inch of perched water is unacceptable. If your pot is 3" deep and the bottom 1/2" of the soil is saturated, that is about 17% of the soil saturated - enough to root roots, impair root function, and cause the marginal or tip necrosis you inquire about. Al

Some grevilleas are really picky when it comes to soil quality. Most of them hate alkaline soil. They also dislike too rich soil, and too much phosphorus may even kill them. Overwatering may also present a problem since they are mostly dry climate plants. The following link gives you some good advice. Good luck with these beauties! I have a few myself. Here is a link that might be useful: Growing Grevillea

Looks like its getting burnt to a crisp. When did you plant it? If it doesn't have any roots set up, it's getting blazed with hot sun, and you fertilized it, I think that would cause the burning. There could be something wacky with your soil too.

For a start I'd make sure the wood chips are away from the base of the trunk on both plants. Also, what sort of wood chips are they? Not sure but they could be your problem. By the way, I copied this from a comment on the plant: 'In order to maintain a healthy garden plant, the conditions must be suitable for propagation. G. rhyolitica requires a well-drained garden bed, with coarse, slightly acidic soil. Borers and scale insects are common pests to grevilleas and can cause considerable damage, leading to death if left unchecked. A minor effect of scale infestation is the growth of black sooty mould. This looks unsightly but is relatively harmless. In some regions of Australia Phytopthera and other diseases threaten both wild and cultivated populations. Most pests can be treated with commercial pesticides'.