Mother Nature's thumbprint!

This rock has a rich, reddish-brown color with swirling, layered patterns and a polished, glossy surface. This suggests it could be a type of petrified wood or a silicified (agate/jasper) material, both of which are common in areas with volcanic or sedimentary history, like parts of Idaho near Boise.
Here’s a breakdown of what it might be:
Petrified Wood: This is fossilized wood where the organic material has been replaced by minerals like quartz, often resulting in colorful, banded patterns. The reddish-brown hues and wood-like grain you see could indicate petrified wood, especially if it was found in an area with ancient forests or volcanic activity. Idaho, including regions near Boise, has deposits of petrified wood, particularly in areas like the Snake River Plain or Owyhee Mountains.
Jasper or Agate: These are forms of chalcedony (a type of quartz) that often have vibrant colors and banding. Jasper tends to be opaque and can have earthy tones like red, brown, or yellow, which match the rock in your image. Agate is similar but often has more translucent bands. If the rock has a waxy or glassy sheen, it could lean toward agate or jasper, both of which are found in Idaho’s volcanic and sedimentary regions.
Other Possibilities: It could also be a type of chert or flint, which are microcrystalline quartz varieties with similar colors and patterns, or even a polished sedimentary rock like banded iron formation, though that’s less likely given the glossy finish.
The location where this rock was found would help narrow it down. Since you mentioned Lucky Peak Reservoir near Boise, the geology of that area includes volcanic rocks, sedimentary deposits, and some metamorphic rocks from the Idaho Batholith. Petrified wood and agates/jaspers are known to occur in Idaho, often in areas with past volcanic activity or river deposits, which aligns with the Boise River and surrounding regions.
The polished appearance suggests it’s been tumbled or treated, which is common for decorative stones like petrified wood, jasper, or agate. If you found this near Lucky Peak or the Boise River, it’s likely a naturally occurring specimen from the area, possibly collected and polished.
To be more certain, you’d need to consider:
The rock’s hardness (petrified wood, jasper, and agate are all around 6.5-7 on the Mohs scale).
Any additional features like weight, texture, or specific patterns.
Whether it reacts to acid (quartz-based rocks like these won’t).