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There is no single hand-sized naturally occurring material that can continuously emit heat over long periods of time. If there was such a material, we would already be using it for power generation, wouldn't we? There are some possibilities, but none are ideal. However, you may be able to twist some of them into your story.

You have four options:

1. Radioactive materials. The three most abundant radioactive elements in Earth's crust are potassium, uranium, and thorium. All three are enriched in certain types of granites. In particular, thorium and uranium are abundant in the minerals monazite and xenotime. These minerals tend to concentrate in mineral sands, so they can be mined, concentrated in put into containers that can contain the heat, and then exploited. This is not something that I would recommend, because (1) it's radioactive, and (2) there isn't that much heat coming out of it.
2. Oxidation. Take for example, pyrite - fool's good. It is iron sulfide, unstable in the atmosphere. By crushing it to finely divided powder you are accelerating the reaction rate with the atmosphere, essentially allowing it to burn. The product is heat, sulfuric acid, and iron rust.
3. Infra-red fluorescence. A material that absorbs energy in the form of visible light, and emits it back again in the IR region, which is then perceived as heat.
4. My favourite: An extra-solar meteorite containing material from a recent supernova. This material will be full of radioactive isotopes: Al-26, various actinides (plutonium, etc etc). It will be hot. The downside, is that it probably doesn't fit your story of "life energy" because handling it will probably cause radiation death very soon.

EDIT

Ok so here's an idea, which would be an elaboration of point 1. A naturally occurring nuclear reactor forms a concentration of fission products. This essentially heat-producing nuclear waste, and it can be mined (as requested in your question).

But, it's radioactive. One way to shield people from the radiation, but allow the heat to come out of it, is by enclosing it in lead glass. This glass would block most of the radiation, with the added benefit of being very pretty.

There is no single hand-sized naturally occurring material that can continuously emit heat over long periods of time. If there was such a material, we would already be using it for power generation, wouldn't we? There are some possibilities, but none are ideal. However, you may be able to twist some of them into your story.

You have four options:

1. Radioactive materials. The three most abundant radioactive elements in Earth's crust are potassium, uranium, and thorium. All three are enriched in certain types of granites. In particular, thorium and uranium are abundant in the minerals monazite and xenotime. These minerals tend to concentrate in mineral sands, so they can be mined, concentrated in put into containers that can contain the heat, and then exploited. This is not something that I would recommend, because (1) it's radioactive, and (2) there isn't that much heat coming out of it.
2. Oxidation. Take for example pyrite, a.k.a "fool's gold". It is iron sulfide, unstable in the atmosphere. By crushing it to finely divided powder you are accelerating the reaction rate with the atmosphere, essentially allowing it to burn. The product is heat, sulfuric acid, and iron rust.
3. Infra-red fluorescence. A material that absorbs energy in the form of visible light, and emits it back again in the IR region, which is then perceived as heat.
4. My favourite: An extra-solar meteorite containing material from a recent supernova. This material will be full of radioactive isotopes: Al-26, various actinides (plutonium, etc etc). It will be hot. The downside, is that it probably doesn't fit your story of "life energy" because handling it will probably cause radiation death very soon.

EDIT

Ok so here's an idea, which would be an elaboration of point 1. A naturally occurring nuclear reactor forms a concentration of fission products. This essentially heat-producing nuclear waste, and it can be mined (as requested in your question).

But, it's radioactive. One way to shield people from the radiation, but allow the heat to come out of it, is by enclosing it in lead glass. This glass would block most of the radiation, with the added benefit of being very pretty.

There is no single hand-sized naturally occurring material that can continuously emit heat over long periods of time. If there was such a material, we would already be using it for power generation, wouldn't we? There are some possibilities, but none are ideal. However, you may be able to twist some of them into your story.

You have four options:

1. Radioactive materials. The three most abundant radioactive elements in Earth's crust are potassium, uranium, and thorium. All three are enriched in certain types of granites. In particular, thorium and uranium are abundant in the minerals monazite and xenotime. These minerals tend to concentrate in mineral sands, so they can be mined, concentrated in put into containers that can contain the heat, and then exploited. This is not something that I would recommend, because (1) it's radioactive, and (2) there isn't that much heat coming out of it.
2. Oxidation. Take for example, pyrite - fool's good. It is iron sulfide, unstable in the atmosphere. By crushing it to finely divided powder you are accelerating the reaction rate with the atmosphere, essentially allowing it to burn. The product is heat, sulfuric acid, and iron rust.
3. Infra-red fluorescence. A material that absorbs energy in the form of visible light, and emits it back again in the IR region, which is then perceived as heat.
4. My favourite: An extra-solar meteorite containing material from a recent supernova. This material will be full of radioactive isotopes: Al-26, various actinides (plutonium, etc etc). It will be hot. The downside, is that it probably doesn't fit your story of "life energy" because handling it will probably cause radiation death very soon.

There is no single hand-sized naturally occurring material that can continuously emit heat over long periods of time. If there was such a material, we would already be using it for power generation, wouldn't we? There are some possibilities, but none are ideal. However, you may be able to twist some of them into your story.

You have four options:

1. Radioactive materials. The three most abundant radioactive elements in Earth's crust are potassium, uranium, and thorium. All three are enriched in certain types of granites. In particular, thorium and uranium are abundant in the minerals monazite and xenotime. These minerals tend to concentrate in mineral sands, so they can be mined, concentrated in put into containers that can contain the heat, and then exploited. This is not something that I would recommend, because (1) it's radioactive, and (2) there isn't that much heat coming out of it.
2. Oxidation. Take for example, pyrite - fool's good. It is iron sulfide, unstable in the atmosphere. By crushing it to finely divided powder you are accelerating the reaction rate with the atmosphere, essentially allowing it to burn. The product is heat, sulfuric acid, and iron rust.
3. Infra-red fluorescence. A material that absorbs energy in the form of visible light, and emits it back again in the IR region, which is then perceived as heat.
4. My favourite: An extra-solar meteorite containing material from a recent supernova. This material will be full of radioactive isotopes: Al-26, various actinides (plutonium, etc etc). It will be hot. The downside, is that it probably doesn't fit your story of "life energy" because handling it will probably cause radiation death very soon.

EDIT

Ok so here's an idea, which would be an elaboration of point 1. A naturally occurring nuclear reactor forms a concentration of fission products. This essentially heat-producing nuclear waste, and it can be mined (as requested in your question).

But, it's radioactive. One way to shield people from the radiation, but allow the heat to come out of it, is by enclosing it in lead glass. This glass would block most of the radiation, with the added benefit of being very pretty.

There is no single hand-sized naturally occurring material that can continuously emit heat over long periods of time. If there was such a material, we would already be using it for power generation, wouldn't we? There are some possibilities, but none are ideal. However, you may be able to twist some of them into your story.

You have four options:

1. Radioactive materials. The three most abundant radioactive elements in Earth's crust are potassium, uranium, and thorium. All three are enriched in certain types of granites. In particular, thorium and uranium are abundant in the minerals monazite and xenotime. These minerals tend to concentrate in mineral sands, so they can be mined, concentrated in put into containers that can contain the heat, and then exploited. This is not something that I would recommend, because (1) it's radioactive, and (2) there isn't that much heat coming out of it.
2. Oxidation. Take for example, pyrite - fool's good. It is iron sulfide, unstable in the atmosphere. By crushing it to finely divided powder you are accelerating the reaction rate with the atmosphere, essentially allowing it to burn. The product is heat, sulfuric acid, and iron rust.
3. Infra-red fluorescence. A material that absorbs energy in the form of visible light, and emits it back again in the IR region, which is then perceived as heat.
4. My favourite: An extra-solar meteorite containing material from a recent supernova. This material will be full of radioactive isotopes: Al-26, various actinides (plutonium, etc etc). It will be hot. The downside, is that it probably doesn't fit your story of "life energy" because handling it will probably cause radiation death very soon.

There is no single hand-sized naturally occurring material that can continuously emit heat over long periods of time. If there was such a material, we would already be using it for power generation, wouldn't we? There are some possibilities, but none are ideal. However, you may be able to twist some of them into your story.

You have four options:

1. Radioactive materials. The three most abundant radioactive elements in Earth's crust are potassium, uranium, and thorium. All three are enriched in certain types of granites. In particular, thorium and uranium are abundant in the minerals monazite and xenotime. These minerals tend to concentrate in mineral sands, so they can be mined, concentrated in put into containers that can contain the heat, and then exploited. This is not something that I would recommend, because (1) it's radioactive, and (2) there isn't that much heat coming out of it.
2. Oxidation. Take for example, pyrite - fool's good. It is iron sulfide, unstable in the atmosphere. By crushing it to finely divided powder you are accelerating the reaction rate with the atmosphere, essentially allowing it to burn. The product is heat, sulfuric acid, and iron rust.
3. Infra-red fluorescence. A material that absorbs energy in the form of visible light, and emits it back again in the IR region, which is then perceived as heat.
4. My favourite: An extra-solar meteorite containing material from a recent supernova. This material will be full of radioactive isotopes: Al-26, various actinides (plutonium, etc etc). It will be hot. The downside, is that it probably doesn't fit your story of "life energy" because handling it will probably cause radiation death very soon.