In a column for The Verge, Sheena Vasani argues that her experience with a $249 UV-tracking necklace called Gem forced a fundamental re-evaluation of how sunlight impacts her health beyond the simple aesthetic of aging. The report details a key August day in Los Angeles where Vasani, despite seeking shade and using a fan, experienced heat exhaustion that the Gem necklace directly linked to high UV exposure, challenging her assumptions about protection.
How a Patio Umbrella Failed and Gem’s Sensor Revealed the Truth
The catalyst for this health awakening was a seemingly straightforward outdoor party. Temperatures soared into the 90s, and Vasani sought refuge under a patio umbrella, believing it provided adequate protection from both heat and the sun. Her assumption, however, was critically flawed. UV radiation isn’t blocked by shade alone; it can be reflected or scattered by surrounding objects. Half an hour later, she felt exhausted, dizzy, and nauseous—a classic case of heat exhaustion.
It was at this moment that The90 Gem, a UV-tracking necklace she had been wearing, sent a notification warning of a significant spike in UV exposure. The device, which costs $249 (or $149 at the time of its initial release), didn’t diagnose her condition but served as a crucial prompt. This led to a discovery: One of my medications increases my sensitivity to heat, which helped to explain similar episodes I’d experienced over the years,
Vasani wrote. The Gem didn’t provide a diagnosis, but it motivated her to investigate a recurring health issue she had previously dismissed.
The Technical Breakdown: UVA, UVB, and a Wearable Sensor
The core of the Gem’s technical capability lies in its separate measurement of UVA and UVB radiation, a distinction that moves it beyond simply displaying a generic UV index forecast. The sensor provides granular data: UVB is primarily responsible for sunburn and is essential for vitamin D synthesis, while UVA penetrates deeper, contributing to photoaging and DNA damage even without causing a visible burn. This separation is a key differentiator for a device in the consumer health space.
The device itself is designed for discreet, all-day wear. Its circular pendant attaches to a thin, stylish titanium chain available in gold or silver, an aesthetic Vasani notes she would appreciate even without its tracking function. The battery life is advertised to last one week, a claim she confirmed, and it recharges via an included magnetic charging dock. The90’s modular design allows the pendant to be detached, and the company has announced plans for future accessories like a hat clip and a bag tag for users who prefer not to wear it as a necklace.
What the App Actually Measures
The mobile application is central to the Gem experience, translating raw sensor data into actionable, jargon-light insights. Users can create a detailed skin profile based on factors like skin type, sensitivities, clothing, and sunscreen use. This profile allows the app to generate a range of estimated metrics beyond simple UV counts. These include:
- Estimated vitamin D production
- Potential circadian rhythm benefits
- Nitric oxide-related benefits
- Cumulative and recent UVA exposure
- A personalized “Skin Aging Zone” with risk assessment and skincare advice
- A dynamic “Sunburn Countdown” and “Sun Score”
The Sun Score, a rating from 1-100, aims to balance the benefits of sunlight against the risks of overexposure. The company encourages users to aim for a daily score of 90, but Vasani found that the individual estimated metrics were more revealing than this aggregated score. The app’s graphs responded quickly to environmental changes, accurately registering shifts between sunlight, shade, and even near windows.
However, a significant usability constraint emerged. Measurements are not stored locally on the device; they rely on the app running in the background. Eventually, I discovered the app needs to remain running in the background (at least on iPhone; I haven’t tested on Android), as measurements aren’t stored on the sensor, and fully closing the app interrupts tracking.
This requirement for constant background activity is a notable limitation that could impact battery life on some devices and frustrate users who prefer to close apps to conserve power.
The Vitamin D Paradox: Melanin’s Dual Role
A profound outcome of using the Gem was a new understanding of her personal vitamin D production. Despite living in perpetually sunny Los Angeles and having a skin type with higher melanin content (which offers some natural UV protection), Vasani found she was consistently falling short of the app’s estimated vitamin D targets. This prompted research into the complex relationship between melanin and UVB absorption.
While melanin provides excellent protection against UVB-induced sunburn, it can also reduce the skin’s ability to synthesize vitamin D from the same rays. This created a personal paradox: she was getting enough sun to potentially cause damage without getting enough of the specific type of radiation (UVB) her body needed to produce sufficient vitamin D. The device did not solve this but made her aware of a nutritional consideration she had never before factored into her sun-exposure habits.
The precision of the app’s countdown timers, however, proved unreliable. The estimated time needed for vitamin D production or the time until a potential sunburn would fluctuate wildly—from several hours down to under an hour, then back up to more than six minutes later. These dramatic shifts, likely tied to rapid changes in UVB intensity, made the exact countdowns difficult to trust. Vasani ultimately stopped focusing on the specific numbers and instead used them as a general gauge of whether her current level of exposure felt appropriate.
The $249 Question: Health Tool or Vanity Tech?
Vasani admits her initial interest in the Gem was largely vanity-driven.
As someone who started going gray in her 20s, she was keen to manage other visible signs of aging. The Gem, however, succeeded in shifting her focus from purely cosmetic concerns to a more holistic view of her physical well-being. Its value became less about preventing wrinkles and more about understanding how her environment was affecting her body on a functional level.
The price remains a significant barrier. At $249, it is a premium device, especially for someone like Vasani who spends considerable time indoors. She notes that she personally can’t justify the price
but sees clear appeal for those who spend extensive time outdoors or are highly mindful of sun protection. The lack of a subscription fee is a positive, but for the cost, she wishes the app’s guidance were clearer and that it didn’t require constant background activity to function.
The90 is expanding its ecosystem.
The 30-Second Verdict on Personal UV Tracking
The Gem UV-tracking necklace from The90 is a device that monitors UVA and UVB radiation. For Vasani, its greatest value wasn’t in preventing wrinkles, but in sparking a fundamental shift toward a more mindful and holistic approach to her personal health in the sun.
